We allow the use of underscored symbols to represent classes throughout
other parts of Rails so it seems incongruous that it's not supported in
`register_interceptor` and `register_observer`.

Since preview_path is read from ActionMailer::Base when previewing, subclasses
can’t change it so don’t there's no need for the extra overhead imposed by using it.

To support the ability for tools like CSS style inliners to operate on emails
being previewed this commit adds a hook in a similar fashion to the existing
delivery interceptor hook, e.g:

  class CSSInlineStyler
    def self.previewing_email(message)
      # inline CSS styles
    end
  end

  ActionMailer::Base.register_preview_interceptor CSSInlineStyler

Fixes #13622.

thread -> threaded [ci skip]

Correct grammar from '... allowing both thread web servers ...' to '... allowing both threaded web servers ...'.

Add a missing changelog entry for #13825 [ci skip]

Fixes #13824

Refactored validations rules for has_secure_password

Remove unused argument.

Fix `ActiveRecord::RecordNotFound` error message with custom primary key

Reordered classes in AR Validation #validates_with example [ci skip]

Person called GoodnessValidator before it was defined.
This change will compile the example correctly.

app_rails_loader.rb should check if bin/rails is a File before calling F...

app_rails_loader.rb should check if bin/rails is a File before calling File.read(exe); closes #13825

Fixes #13810 `rake routes` error when mount `Rails::Engine` with empty routes

Squash

Remove unused assignment to fix warnings in enum test.

unify param.require tests

test boolean and number json param parsing

Fix ActiveModel::Errors#has_key? returning nil

Better ActiveRecord hierarchy for Dirty and others

Move serialization dirty into serialization.rb

Remove duplicate merge