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

Offer the option to use parameterization for shared processing of headers and ivars

(I personally prefer writing one string in one line no matter how long it is, though)

Actually, private methods cannot be called with `self.`, so it's not just redundant, it's a bad habit in Ruby

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.

Usually users extends tests classes doing something like:

    ActionView::TestCase.include MyCustomTestHelpers

This is bad because it will load the ActionView::TestCase right aways
and this will load ActionController::Base making its on_load hooks to
execute early than it should.

One way to fix this is using the on_load hooks of the components like:

    ActiveSupport.on_load(:action_view) do
      ActionView::TestCase.include MyCustomTestHelpers
    end

The problem with this approach is that the test extension will be only
load when ActionView::Base is loaded and this may happen too late in the
test.

To fix this we are adding hooks to people extend the test classes that
will be loaded exactly when the test classes are needed.

Style/SpaceBeforeBlockBraces
Style/SpaceInsideBlockBraces
Style/SpaceInsideHashLiteralBraces

Fix all violations in the repository.

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`.