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

`ActionMailer::Base.delivery_method` is already defined in
https://github.com/rails/rails/blob/master/actionmailer/test/parameterized_test.rb#L13

Adding durations to `Time` instances is perfectly okay.

Currently `ActiveSupport::Duration` implicitly converts to a seconds
value when used in a calculation except for the explicit examples of
addition and subtraction where the duration is the receiver, e.g:

    >> 2 * 1.day
    => 172800

This results in lots of confusion especially when using durations
with dates because adding/subtracting a value from a date treats
integers as a day and not a second, e.g:

    >> Date.today
    => Wed, 01 Mar 2017
    >> Date.today + 2 * 1.day
    => Mon, 10 Apr 2490

To fix this we're implementing `coerce` so that we can provide a
deprecation warning with the intent of removing the implicit coercion
in Rails 5.2, e.g:

    >> 2 * 1.day
    DEPRECATION WARNING: Implicit coercion of ActiveSupport::Duration
    to a Numeric is deprecated and will raise a TypeError in Rails 5.2.
    => 172800

In Rails 5.2 it will raise `TypeError`, e.g:

    >> 2 * 1.day
    TypeError: ActiveSupport::Duration can't be coerced into Integer

This is the same behavior as with other types in Ruby, e.g:

    >> 2 * "foo"
    TypeError: String can't be coerced into Integer
    >> "foo" * 2
    => "foofoo"

As part of this deprecation add `*` and `/` methods to `AS::Duration`
so that calculations that keep the duration as the receiver work
correctly whether the final receiver is a `Date` or `Time`, e.g:

    >> Date.today
    => Wed, 01 Mar 2017
    >> Date.today + 1.day * 2
    => Fri, 03 Mar 2017

Fixes #27457.

[ci skip]

ActionMailer::Base#instrument_name and
ActionController::Base#instrument_name will be frequently called once
caching is enabled. So it's better to freeze them instead of create new
string on every call.

Also, the instrument name in #instrument_fragment_cache will usually
be "write_fragment.action_controller" or
"read_fragment.action_controller". So freezing them might also gain some
performance improvement. We have done something like this in other places:
https://github.com/rails/rails/blob/master/actionview/lib/action_view/template.rb#L348

The raw_source method is documented as returning the exact value that
was used to create the body; mutating it breaks that contract.

Additionally, if the value used to create the body is blank, raw_source
returns a frozen string which causes the interceptor to raise an error.

Also change the class_methods to ClassMethods since the former document
the method as an instance method of Parameterized not as a class method.

We own the class so we can override the initialize.

Needed for testing of parameterized mailers