/cc @pixeltrix

No need to do this in railtie as AM depends on AV either way

This behavior is documented in our guides (http://edgeguides.rubyonrails.org/action_mailer_basics.html#action-mailer-callbacks)
but was broken in the past. This commit short curcuits
the `mail` method if:

  1. mail() was previously called
  2. no headers are passed
  3. no block is passed

Closes #13090.

/cc @pixeltrix

constant.

`view_assigns` can use the precalculated sets and remove instance
variables without allocating any extra arrays

The processing of outbound mail is instrumented with the key
`process.action_mailer`. The payload includes the mailer name as well as
the mailer method.

This reverts commit 7de994fa.

It's already included in Layouts, makes no sense to doubly include it

Invoke mailer defaults as procs only if they are procs, do not convert
with to_proc.  That an object is convertible to a proc does not mean it's
meant to be always used as a proc.  Fixes #11533

This reverts commit 328e876a.

Reason: Let's not nodoc without reason.

[ci skip]

ActionMailer templates that are not plain text do not require *.text.* in the name and will fail to be picked up automatically if it is included in the name.

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