Replace AR with ActiveRecord to make it more readable [ci skip]

Add tests for test/cases/adapters/mysql2/view_test.rb

Basically view tests for MySQL are same with
`test/cases/adapters/postgresql/view_test.rb`.

So move `test/cases/adapters/postgresql/view_test.rb` to
`test/cases/view_test.rb` and make them only run if
`current_adapter` supports writable view.

raise LoadError when a non-existent file or directory is specified to the test runner

Its value never change since associations are defined at class load time
so there is no need to build the hash everytime the method is called.

Before this change:

    Calculating -------------------------------------
             reflections   804.000  i/100ms
    -------------------------------------------------
             reflections      8.213k (±26.2%) i/s -     36.180k

After this change:

    Calculating -------------------------------------
             reflections    24.548k i/100ms
    -------------------------------------------------
             reflections      1.591M (±25.7%) i/s -      7.364M

Benchmark script:

    require 'bundler/setup'

    require 'active_record'
    require 'benchmark/ips'

    ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
    ActiveRecord::Migration.verbose = false

    ActiveRecord::Schema.define do
      100.times do |i|
        create_table "users#{i}", force: true
      end

      create_table :cars, force: true do |t|
        100.times do |i|
          t.references "users#{i}"
        end
      end
    end

    class Car < ActiveRecord::Base
      100.times do |i|
        belongs_to "users#{i}".to_sym
      end
    end

    Benchmark.ips do |x|
      x.report('reflections') { Car.reflections }
    end

PERF: Reduce allocation in `resolve_column_aliases`.

Benchmark Script Used:
```
begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  gem 'rails', path: '~/rails' # master against ref "f1f0a3f8"
  gem 'arel', github: 'rails/arel', branch: 'master'
  gem 'rack', github: 'rack/rack', branch: 'master'
  gem 'sass'
  gem 'sprockets-rails', github: 'rails/sprockets-rails', branch: 'master'
  gem 'sprockets', github: 'rails/sprockets', branch: 'master'
  gem 'pg'
  gem 'benchmark-ips'
end

require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection('postgres://postgres@localhost:5432/rubybench')

ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name, :email
    t.timestamps null: false
  end
end

class User < ActiveRecord::Base; end

attributes = {
  name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.",
  email: "foobar@email.com",
}

1000.times { User.create!(attributes) }

Benchmark.ips(5, 3) do |x|
  x.report('where with hash single') { User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.") }
  x.report('where with string single') { User.where("users.name = ?", "Lorem ipsum dolor sit amet, consectetur adipiscing elit.") }
  x.report('where with hash double') { User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.", email: "foobar@email.com") }
  x.report('where with string double') { User.where("users.name = ? AND users.email = ?", "Lorem ipsum dolor sit amet, consectetur adipiscing elit.", "foobar@email.com") }
  x.compare!
end
```

Before:
```
Calculating -------------------------------------
where with hash single
                         3.300k i/100ms
where with string single
                         4.965k i/100ms
where with hash double
                         2.594k i/100ms
where with string double
                         4.400k i/100ms
-------------------------------------------------
where with hash single
                         35.161k (± 1.2%) i/s -    178.200k
where with string single
                         53.368k (± 2.9%) i/s -    268.110k
where with hash double
                         27.364k (± 1.1%) i/s -    137.482k
where with string double
                         46.876k (± 2.1%) i/s -    237.600k

Comparison:
where with string single:    53368.1 i/s
where with string double:    46875.5 i/s - 1.14x slower
where with hash single:    35160.8 i/s - 1.52x slower
where with hash double:    27364.0 i/s - 1.95x slower
```

After:
```
Calculating -------------------------------------
where with hash single
                         3.403k i/100ms
where with string single
                         5.167k i/100ms
where with hash double
                         2.659k i/100ms
where with string double
                         4.597k i/100ms
-------------------------------------------------
where with hash single
                         36.410k (± 1.3%) i/s -    183.762k
where with string single
                         55.009k (± 2.6%) i/s -    279.018k
where with hash double
                         27.951k (± 1.4%) i/s -    140.927k
where with string double
                         48.362k (± 2.6%) i/s -    243.641k

Comparison:
where with string single:    55008.6 i/s
where with string double:    48361.5 i/s - 1.14x slower
where with hash single:    36410.1 i/s - 1.51x slower
where with hash double:    27950.9 i/s - 1.97x slower
```

Make `config.force_ssl` less dangerous to try and easier to disable

SSL redirect:
* Move `:host` and `:port` options within `redirect: { … }`. Deprecate.
* Introduce `:status` and `:body` to customize the redirect response.
  The 301 permanent default makes it difficult to test the redirect and
  back out of it since browsers remember the 301. Test with a 302 or 307
  instead, then switch to 301 once you're confident that all is well.

HTTP Strict Transport Security (HSTS):
* Shorter max-age. Shorten the default max-age from 1 year to 180 days,
  the low end for https://www.ssllabs.com/ssltest/ grading and greater
  than the 18-week minimum to qualify for browser preload lists.
* Disabling HSTS. Setting `hsts: false` now sets `hsts: { expires: 0 }`
  instead of omitting the header. Omitting does nothing to disable HSTS
  since browsers hang on to your previous settings until they expire.
  Sending `{ hsts: { expires: 0 }}` flushes out old browser settings and
  actually disables HSTS:
    http://tools.ietf.org/html/rfc6797#section-6.1.1
* HSTS Preload. Introduce `preload: true` to set the `preload` flag,
  indicating that your site may be included in browser preload lists,
  including Chrome, Firefox, Safari, IE11, and Edge. Submit your site:
    https://hstspreload.appspot.com

Support mysql2 0.4.0, first release with prepared statements support

Known failure on Ruby 2.3/trunk: brianmario/mysql2#671

test runner sets file to be tested in plugin_rails_options,
but in plugin_rails_options, processing has been made to the argument of the
actual command rather than the argument of Minitest.run.

For example, if you run `./bin rake db:migrate test`, the options[:patterns], `db:migrate test` was incorrectly set.

Related with #20418

[Rails Guides] Clarify custom code configuration [ci skip]

This reverts commit 51dd2588, reversing
changes made to ecb4e4b2.

This broke Active Record tests

Validate multiple contexts on `valid?` and `invalid?` at once

PERF: Scope performance.

Generic cast-to-text was only added in 8.3.

PERF: Don't create a Relation when it is not needed.

Typo fix (ci skip)

`lengh` should be `length`  

Add require to ensure Time#advance works without implicit required

Use global migrations_path configuration in Migrator

Fix strange messages for `rails g foo`

Benchmark script used:
```
begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  gem 'rails', path: '~/rails' # master against ref "f1f0a3f8"
  gem 'arel', github: 'rails/arel', branch: 'master'
  gem 'rack', github: 'rack/rack', branch: 'master'
  gem 'sass'
  gem 'sprockets-rails', github: 'rails/sprockets-rails', branch: 'master'
  gem 'sprockets', github: 'rails/sprockets', branch: 'master'
  gem 'pg'
  gem 'benchmark-ips'
end

require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection('postgres://postgres@localhost:5432/rubybench')

ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name, :email
    t.timestamps null: false
  end
end

class User < ActiveRecord::Base; end

attributes = {
  name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.",
  email: "foobar@email.com",
}

1000.times { User.create!(attributes) }

Benchmark.ips(5, 3) do |x|
  x.report('all') { User.all }
end

key =
  if RUBY_VERSION < '2.2'
    :total_allocated_object
  else
    :total_allocated_objects
  end

before = GC.stat[key]
User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.")
after = GC.stat[key]
puts "Total Allocated Object: #{after - before}"

```

Before:
```
Calculating -------------------------------------
                 all    17.569k i/100ms
-------------------------------------------------
                 all    190.854k (± 3.3%) i/s -    966.295k
Total Allocated Object: 85
```

After:
```
Calculating -------------------------------------
                 all    22.237k i/100ms
-------------------------------------------------
                 all    262.715k (± 5.5%) i/s -      1.312M
Total Allocated Object: 80
```

safe_constantize - Added Object scoped missing test cases

adding description of return value [ci skip]

Benchmark Script:
```
begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  # gem 'rails', github: 'rails/rails', ref: 'f1f0a3f8'
  gem 'rails', path: '~/rails'
  gem 'arel', github: 'rails/arel', branch: 'master'
  gem 'rack', github: 'rack/rack', branch: 'master'
  gem 'sass'
  gem 'sprockets-rails', github: 'rails/sprockets-rails', branch: 'master'
  gem 'sprockets', github: 'rails/sprockets', branch: 'master'
  gem 'pg'
  gem 'benchmark-ips'
end

require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection('postgres://postgres@localhost:5432/rubybench')

ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name, :email
    t.timestamps null: false
  end
end

class User < ActiveRecord::Base; end

attributes = {
  name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.",
  email: "foobar@email.com",
}

1000.times { User.create!(attributes) }

Benchmark.ips(5, 3) do |x|
  x.report('where with hash') { User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.") }
  x.report('where with string') { User.where("users.name = ?", "Lorem ipsum dolor sit amet, consectetur adipiscing elit.") }
  x.compare!
end

key =
  if RUBY_VERSION < '2.2'
    :total_allocated_object
  else
    :total_allocated_objects
  end

before = GC.stat[key]
User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.")
after = GC.stat[key]
puts "Total Allocated Object: #{after - before}"
```

Stackprof output truncated.
```
TOTAL    (pct)     SAMPLES    (pct)     FRAME
   52  (10.6%)          10   (2.0%)     ActiveRecord::Scoping::Default::ClassMethods#build_default_scope
```

Before:
```
Calculating -------------------------------------
     where with hash     2.789k i/100ms
   where with string     4.407k i/100ms
-------------------------------------------------
     where with hash     29.170k (± 1.9%) i/s -    147.817k
   where with string     46.954k (± 2.7%) i/s -    237.978k

Comparison:
   where with string:    46954.3 i/s
     where with hash:    29169.9 i/s - 1.61x slower

Total Allocated Object: 85
Calculating -------------------------------------
                 all    16.773k i/100ms
-------------------------------------------------
                 all    186.102k (± 3.6%) i/s -    939.288k
```

After:
```
Calculating -------------------------------------
     where with hash     3.014k i/100ms
   where with string     4.623k i/100ms
-------------------------------------------------
     where with hash     31.524k (± 1.3%) i/s -    159.742k
   where with string     49.948k (± 2.3%) i/s -    249.642k

Comparison:
   where with string:    49948.3 i/s
     where with hash:    31524.3 i/s - 1.58x slower

Total Allocated Object: 84
Calculating -------------------------------------
                 all    20.139k i/100ms
-------------------------------------------------
                 all    227.860k (± 2.5%) i/s -      1.148M
```

Stackprof output truncated.
```
TOTAL    (pct)     SAMPLES    (pct)     FRAME
  23   (4.7%)          12   (2.4%)     Hash#transform_keys
  11   (2.2%)          11   (2.2%)     block in Hash#transform_keys
  30   (6.1%)           7   (1.4%)     Hash#stringify_keys
```

Benchmark Script:
```
begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  gem 'rails', path: '~/rails' # master against ref "f1f0a3f8"
  gem 'arel', github: 'rails/arel', branch: 'master'
  gem 'rack', github: 'rack/rack', branch: 'master'
  gem 'sass'
  gem 'sprockets-rails', github: 'rails/sprockets-rails', branch: 'master'
  gem 'sprockets', github: 'rails/sprockets', branch: 'master'
  gem 'pg'
  gem 'benchmark-ips'
end

require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection('postgres://postgres@localhost:5432/rubybench')

ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name, :email
    t.timestamps null: false
  end
end

class User < ActiveRecord::Base; end

attributes = {
  name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.",
  email: "foobar@email.com",
}

1000.times { User.create!(attributes) }

Benchmark.ips(5, 3) do |x|
  x.report('where with hash') { User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.") }
  x.report('where with string') { User.where("users.name = ?", "Lorem ipsum dolor sit amet, consectetur adipiscing elit.") }
  x.compare!
end

key =
  if RUBY_VERSION < '2.2'
    :total_allocated_object
  else
    :total_allocated_objects
  end

before = GC.stat[key]
User.where(name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.")
after = GC.stat[key]
puts "Total Allocated Object: #{after - before}"
```

Before:
```
Calculating -------------------------------------
     where with hash     2.796k i/100ms
   where with string     4.338k i/100ms
-------------------------------------------------
     where with hash     29.177k (± 1.5%) i/s -    148.188k
   where with string     47.419k (± 2.8%) i/s -    238.590k

Comparison:
   where with string:    47419.0 i/s
     where with hash:    29176.6 i/s - 1.63x slower

Total Allocated Object: 85
```

After:
```
Calculating -------------------------------------
     where with hash     2.895k i/100ms
   where with string     4.416k i/100ms
-------------------------------------------------
     where with hash     30.758k (± 2.0%) i/s -    156.330k
   where with string     47.708k (± 2.6%) i/s -    238.464k

Comparison:
   where with string:    47707.9 i/s
     where with hash:    30757.7 i/s - 1.55x slower

Total Allocated Object: 84
```

[ci skip] Replace `AR` with `Active Record` in task desc

Many user look `desc` of rake task and they are not familiar with `AR`.
`Active Record` is more familiar word.

Currently, if a file or directory that does not exist was specified in the test runner,
that argument is ignored.
This commit has been modified to cause an error if there is no file or directory.

Apparently I managed to forget how similar the "tests passing" and
"no status reported" merge indicators look.

Note that the previous `stubs` in test_add_index wasn't working:
the method was still called, and just happened to return false.

Support dropping indexes concurrently in Postgres