Notation with `self` was sugestted at 0b05acd4, when self.defaults was
extlib_inheritable_accessor. But now all Action Mailer examples are
without this self. It's nicer.

Injected at a1639ad7.

    # config/locales/en.yml
    en:
      user_mailer:
        welcome:
          subject: 'Hello, %{username}'

    # app/mailers/user_mailer.rb
    class UserMailer < ActionMailer::Base
      def welcome(user)
        mail(subject: default_i18n_subject(username: user.name))
      end
    end

As issue #7978, the order in which ActionMailer
sends multipart messages could be unintentionally
overwritten if a block is passed to the mail
method. This changes the mail method such that
:parts_order is always respected, regardless of
whether a block is passed to mail.

This reverts commit 7e0cf563.

Conflicts:
	actionmailer/CHANGELOG.md

See discussion at
https://github.com/rails/rails/commit/7e0cf563639bc7508da381b1b8321c7a89be1aa8#commitcomment-2075489

This makes it easy to write e.g. `Mailer.expects(:deliver_foo)` when
testing code that calls the mailer.

This reverts commit f5654e78.

Conflicts:
	actionmailer/lib/action_mailer/base.rb

Reason: @frodsan asked me to revert since this change is breaking the
documentation in the edge API site

Use synchronous queue by default. Separate queued message delivery jobs from the queued message wrappers so the queue itself needn't be marshaled (due to queue reference QueuedMessage).

This allow the users to do:

    config.action_mailer.queue = MyQueue.new

and

    class UsersMailer < ActionMailer::Base
      self.queue = MyQueue.new
    end

Fixes warnings when executing rake test in ActionMailer. Related to 582a7f45

Reverting because it feels backward to specify a delivery to not
be performed while the e-mail is being composed. It is simpler (and
makes more sense) to delegate the responsibility to the calling code.

[ci skip]

Allows to easily set :from, :replay_to, etc. options in
config/application.rb using simple syntax:

  config.action_mailer.default_options = {from:"no-replay@example.org"}

This was not possible using #default method because

  config.action_mailer.default(from: "no-replay@example.org")

is interpreated as reader method and just returns nil.
It would not call ActionMailer::Base.default method. The only
way of calling this method from config/application.rb was to use
the direct syntax which looks ugly in my opinion:

  config.assets.enabled = false
  config.assets.version = '1.0'
  config.encoding = "utf-8"
  config.action_mailer.default_url_options= {
    host:"example.org",
    protocol:"https"
  }
  ActionMailer::Base.default(from: "no-replay@example.org")