If the through record gets created in an `after_create` hook that is
defined before the association is defined (therefore after its
`after_create` hook) get saved twice. This ensures that the through
records are created only once, regardless of the order of the hooks.

Move test teardown into `ensure` block.

[ci skip] doc corrected : ActiveRecord::Base.connections to ActiveRecord::Base.configurations

Promote time zone aware attributes to a first class type decorator

This refactoring revealed the need for another form of decoration, which
takes a proc to select which it applies to (There's a *lot* of cases
where this form can be used). To avoid duplication, we can re-implement
the old decoration in terms of the proc-based decoration.

The reason we're `instance_exec`ing the matcher is for cases such as
time zone aware attributes, where a decorator is defined in a parent
class, and a method called in the matcher is overridden by a child
class. The matcher will close over the parent, and evaluate in its
context, which is not the behavior we want.

Refactor application of type decorators into an object

[ci skip] /javascript/ ~> JavaScript

[ci skip] add API doc for AR Group.

[ci skip] correct doc for serialization in accordance with API Doc Guide

Refactor in-place dirty checking to use the attribute object

[ci skip]

Add test cases for Hash#asset_valid_keys

Skip migration check if adapter doesn't support it

Change Http::Cache::SPECIAL_KEYS from Array to Set

Remove unused parameter.

[ci skip] /mysql/i -> MySQL, Spell correct in continuation to #15555

[ci skip] Use `an` for SQL

Correct typo, add test for validates_absence_of

Slightly improves performance, for example, a simple benchmark:

```ruby
require 'benchmark/ips'
require 'set'

SPECIAL_KEYS = %w[extras no-cache max-age public must-revalidate]
SPECIAL_KEYS_SET = Set.new(SPECIAL_KEYS)
directive = 'must-revalidate'

Benchmark.ips do |x|
  x.report('array') { SPECIAL_KEYS.include?(directive) }
  x.report('set') { SPECIAL_KEYS_SET.include?(directive) }
end
```

Output:

```
-------------------------------------
   array     67926 i/100ms
     set     74054 i/100ms
-------------------------------------
   array  2318423.4 (±2.8%) i/s -   11615346 in   5.014899s
     set  3387981.8 (±4.7%) i/s -   16958366 in   5.019355s
```

Remove unused param 'separators' from RouteSet#build_path

Change the deprecation warning on `serialized_attributes`

to "without replacement"

This was partially broken because `preview_interceptors=` just assigned the
raw values, whithout going through `register_preview_interceptor`. Now the
Action Mailer railtie takes care of the `preview_interceptors` option.

This commit is a partial revert of:

Revert "Merge pull request #15739 from y-yagi/correct_doc_for_action_mailer_base"

This reverts commit a15704d7, reversing
changes made to 1bd12a86.

/cc @kuldeepaggarwal @y-yagi

[ci skip] correct doc for `register_preview_interceptor`

remove unnecessary calling of `I18n.backend.store_translations('empty', {})`

Clean up I18n stored translations after test
continuation with #15714