Cuts down on the duplicated reading parts.

Gets rid of the option parsing and makes what the encryptor does stand out.

Lets us avoid worrying about parsing the options and doing just what we need.

Remove the clutter to make PermanentCookieJar's one change stand out.

Eventually this will be the superclass of all the chained jars.

`url_for` does not modify polymorphic options

Fixed Time conversion example for UTC time zone [ci skip]

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

[Rails Guides] clarify `ActiveRecord::ConnectionTimeoutError`

Removes mandatory arguments from AR exceptions

Define `SchemaStatements#tables` as interface

These 3 methods expect `ConnectionAdapters` to have `tables` method,
so make it clear that `tables` method is interface.

* `ConnectionAdapters::SchemaCache#prepare_tables`
* `db:schema:cache:dump` task
* `SchemaDumper#tables`

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.

Example:

```ruby
class Person
  include ActiveModel::Validations

  attr_reader :name, :title
  validates_presence_of :name, on: :create
  validates_presence_of :title, on: :update
end

person = Person.new
person.valid?([:create, :update])    # => true
person.errors.messages               # => {:name=>["can't be blank"], :title=>["can't be blank"]}
```

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