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.

Improve how mailer tests to resolve mailers from the test name.
Add tests for mailer tests using the minitest spec DSL.

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