* Bump RAILS_VERSION
* Bundle
* rake update_versions
* rake changelog:header

It isn't allowed to execute `assert_enqueued_email_with` without block
See #33243

We might allow to do it in Rails 6.0. See #33258.

* Eager autoload mail gem when eager load is true

We had a production issue where our Sidekiq worker threads all became
deadlocked while autoloading a file within the mail gem, required via
ActionMailer, despite setting our Rails applicaiton to eager load.
`Mail.eager_autoload!` exists and works great, ActionMailer just doesn't
call it during eager loading. Adding it to the ActionMailer Railtie's
eager_load_namespaces takes care of calling `Mail.eager_autoload!`
during the `eager_load!` initializer.

* 'Mail' isn't defined yet, use before_eager_load instead

* Make sure mail is loaded

* Move eager load of Mail into ActionMailer.eager_load!

[Samuel Cochran + Rafael Mendonça França]

[ci skip]

[ci skip]

- And move the Active Job related section down. Otherwise it was
  appearing as if the options are available for the `delivery_job`
  setting.

Make clear that the files are not to be run for interpreters.

Fixes #23847.
Fixes #30690.
Closes #23878.

This basically reverts cd9cc721

PR #29270 changed the number of arguments that gets passed to Procs
defined in ActionMail::Base.default. With this changeset, Procs can
now have 1 or 0 arguments

Also adds test coverage for AM::Base.default Proc arity.

The comment was describing a previous version of the method with
a different signature. This is outdated since e76c38ef

On the first request, ActionController::Base#action_methods computes
and memoized the list of available actions [1]. With this PR we move
this expensive operation into eager load step to reduce response time
of the first request served in production.

This also reduces the memory footprint when running on forking server
like Unicorn.

[1] https://github.com/rails/rails/blob/a3813dce9a0c950a4af7909111fa730a2622b1db/actionpack/lib/abstract_controller/base.rb#L66-L77

This reverts commit 3420a145, reversing
changes made to afb66a5a.

Setting delivery_job on a mailer class will cause MessageDelivery to use
the specified job instead of ActionMailer::DeliveryJob:

    class MyMailer < ApplicationMailer
      self.delivery_job = MyCustomDeliveryJob

      ...
    end

* Allow a default value to be declared for class_attribute

* Convert to using class_attribute default rather than explicit setter

* Removed instance_accessor option by mistake

* False is a valid default value

* Documentation

".. with __dir__ we can restore order in the Universe." - by @fxn

Related to 5b8738c2

Without this, Action Mailer doesn't work like it used to on version 4.
The following snippet fails since version 5:

    > require 'action_mailer'
    > ActionMailer::Base
    NameError: uninitialized constant ActiveSupport::Rescuable