create a singleton content type that just has nils, so that we don't
have to allocate a content type object all the time.

If someone sets just a charset, but depends on the implicit type from
rendering, this will store a strange content type header that looks like
this:  `; charset=blah`.  This is so that when the content type header
is parsed again, it will return nil for the actual type.

It turns out that the response object never really cares what the mime
type object is, so just use the string.

pull content-type setting to a private method to dry it up.

Instead of storing content type information in an ivar and a header,
lets move to just store the content type info in just the header.

we'll use this method later to lazily parse content type headers.

everything above metal really doesn't care about setting the content
type, so lets rearrange these methods to be in metal.

_set_content_type only does something when there is a request object,
otherwise the return value of _get_content_type is always ignored. This
commit moves everything to the module that has access to the request
object so we'll never to_s unless there is a reason

in the future I would like to make the header hash read only (or at
least remove guarantees that mutations will do anything).

Properly log nested parameters to Active Job

Refactor arguments logging method for Active Job

Fix typo in activemodel changelog [ci skip]

It's only used there.

`CookieJar` is only at the start of the chain and has its own
request method, so we don't need it in the module.

It was the same in both legacy versions of the signed and encrypted cookie jars.

The `EncryptedCookieJar` already calls it for us, so just delegate to its `parse` implementation.

`SignedCookieJar`'s parse method already attempts to verify the message,
so we can just call super and try the old verifier if it fails.

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