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.

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

Deprecate passing conditions to AR::Relation destroy_all and delete_all methods

Removed Extra ‘the’ [ci skip]

`hfvalue` parts should always be percent encoded, so lets do that!

Revert "use path escaping for email addresses"

This reverts commit 21ffef38.

Remove mocha from ActionPack tests

See http://www.postgresql.org/docs/9.4/static/sql-dropindex.html
for more details.

converts old ID methods to the new abstract store methods in Rack

Just include the modules necessary in the Request object to implement
the things we need.  This should make it easier to build delegate
request objects because the API is smaller

#where fails if opts.responds_to?(:==) unexpectedly

Sometimes opts passed in might respond to ==, e.g. `Arel::Nodes::Grouping`. In this case, `opts == :chain` returns `Arel::Nodes::Equality` which causes odd behaviour. Prefer `if :chain == opts` which guarantees that `Symbol#==` would be invoked. Alternatively consider `eql?`.

Due to e25fdad2, we are correctly using
path escaping for email addresses.  This commit fixes the tests to
expect path escaping.