- Auto-generating content from the filename of an image is not suitable
  alternative text; alt text that isn't fully considered can be
  distracting and fatiguing for screen readers users (blind, low vision,
  dyslexic people).
- Setting a filename fallback short circuits screen reader default
  behavior and configuration for blank descriptions.
- Setting poor defaults also creates false negatives for accessibility
  linting and testing software, that makes it harder to improve
  application accessibility.

***

- After this change, if authors leave images without alt text, screen
  readers will fallback to default behavior for missing alt text.
- Also with this change, Automated linting and testing tools will
  correctly generate warnings.

[Fixes #30096]

If clear it before the test, the mail of the last executed test will not
be correctly cleared.

Therefore, executing the test with seed below will result in an error.

```
./bin/test -w --seed 55480
Run options: --seed 55480

# Running:

...........................................................................................................................................................F

Failure:
MailDeliveryTest#test_does_not_increment_the_deliveries_collection_on_error [/home/yaginuma/program/rails/master_y_yagi/rails/actionmailer/test/delivery_methods_test.rb:221]:
--- expected
+++ actual
@@ -1 +1 @@
-[]
+[#<Mail::Message:47011389364640, Multipart: false, Headers: <Date: Mon, 14 Aug 2017 07:48:40 +0900>, <From: test-sender@test.com>, <To: test-receiver@test.com>, <Message-ID: <5990d748ea5b2_29342ac1af8bcf40886f7@yaginuma.mail>>, <Subject: Test Subject>, <Mime-Version: 1.0>, <Content-Type: text/plain>, <Content-Transfer-Encoding: 7bit>>]

bin/test test/delivery_methods_test.rb:216
```

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 This is outdated since e76c38ef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

This will avoid to the test fail when running in isolation because the
message generated is nil.

`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.