All indentation was normalized by rubocop auto-correct at 80e66cc4.
But comments was still kept absolute position. This commit aligns
comments with method definitions for consistency.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

Follow up to #24436

SSL_set_verify(3) explains:

SSL_VERIFY_FAIL_IF_NO_PEER_CERT
  Server mode: if the client did not return a certificate, the TLS/SSL
handshake is immediately terminated with a "handshake failure" alert.
This flag must
  be used together with SSL_VERIFY_PEER.

  Client mode: ignored

SSL_VERIFY_CLIENT_ONCE
  Server mode: only request a client certificate on the initial TLS/SSL
handshake. Do not ask for a client certificate again in case of a
renegotiation.
  This flag must be used together with SSL_VERIFY_PEER.

  Client mode: ignored

The SMTP connection here uses a OpenSSL socket in client mode,
suggesting invalid/ignored flags is rather misleading.

Follows the same pattern as controllers and jobs. Exceptions raised in
delivery jobs (enqueued by `#deliver_later`) are also delegated to the
mailer's rescue_from handlers, so you can handle the DeserializationError
raised by delivery jobs:

```ruby
class MyMailer < ApplicationMailer
  rescue_from ActiveJob::DeserializationError do
    …
  end
```

ActiveSupport::Rescuable polish:
* Add the `rescue_with_handler` class method so exceptions may be
  handled at the class level without requiring an instance.
* Rationalize `exception.cause` handling. If no handler matches the
  exception, fall back to the handler that matches its cause.
* Handle exceptions raised elsewhere. Pass `object: …` to execute
  the `rescue_from` handler (e.g. a method call or a block to
  instance_exec) against a different object. Defaults to `self`.

Abstract Controller is the common component between Action Mailer and
Action Controller so if we need to share the caching component it need
to be there.

Remove useless helper in ActionDispatch::Caching and fix indentation

For more information about GNU zip mime type please check IETF's web site [RFC6713](http://tools.ietf.org/html/rfc6713) or [IANA](http://www.iana.org/assignments/media-types/media-types.xhtml#application)

This reverts commit 8417d967.

In 5697bdbb and af3eb596,
add mailer suffix to generated files and classes.
Therefore, no longer need to specify `Mailer` to class name. [ci skip]

order to generate the proper files. Some of the docs/comments are
missing this important detail.

Since they are nodoc there is no need to be protected.

We are already filtering the keys in the assign_headers_to_message
method so we can just update the filter.

This reverts commit 65a61ab7, reversing
changes made to 14314ca1.

This PR broke the build

this allows us to construct mailer objects without possibly disastrous
side-effects.