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
```

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

reload I18n as we are defining `:ts` locale  in  `test_number_to_human_with_custom_translation_scope`

fix build- see https://travis-ci.org/rails/rails/jobs/27578159#L2913.

[ci skip] update wiki link for MVC in README

[ci-skip] correct wikipedia MVC link

Deprecate `serialized_attributes` without replacement

Clean up I18n stored translations after test.

This application always gets out of sync and doesn't add much value.

Closes #15721

/cc @rafaelfranca

[ci skip] Update #has_secure_password docs

We've stopped using it internally, in favor of polymorphism. So should
you!

Use `@existing_user` while updating existing user, fixing - ee4e86

Avoid hard-coded value in test setup and teardown.

Make sure restoration always happen.

* Grouped the valid test cases in one place
* Make the length of the generated password obvious
* Removed two wrong (copy-and-pasted) test cases

SecurePassword - Validate password must be less than or equal to 72

See #14591, Reason - BCrypt hash function can handle maximum 72 characters.

Fixes #15705.

Remove `serialized?` from the type interface

Fix request's path_info when a rack app mounted at '/'.

Open extension point for defining options in build_through_record

Remove unused `method_missing` definition

This fixes #15496