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

* 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]

* Add missing credit
* Add backticks
* Fix indentation
* Remove trailing spaces

And some minor tweaks.

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.

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

[ci skip]

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

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`.

- Fixed statement about setting `config.action_mailer.default_url_options = {protocol: 'https'}` . We are just setting the protocol key to 'https', not replacing/initializing the complete config.
- Fixed grammar in assert_emails changlog
- Added sentence separator for code ":"

[ci skip]

They would be lost when the delivery job is enqueued, otherwise.
Prevents a common, hard-to-find bug like:

```ruby
message = Notifier.welcome(user, foo)
message.message_id = my_generated_message_id
message.deliver_later
```

The message_id is silently lost here! *Only the mailer arguments are
passed to the delivery job.*

This raises an exception now.

Make modifications to the message within the mailer method or use a
custom Active Job to manage delivery instead of using #deliver_later.

Removes `-t`

Adds changelog headers for beta3 release

Whenever you are sending emails in integration tests using the `:test`
delivery method you need to make sure that
`ActionMailer::Base.deliveries` is reset after every test. This piece of
boilerplate code is present in all my applications that send
emails. Let's have `ActionDispatch::IntegrationTest` reset the
deliveries automatically.

[ci skip]

During the `5.0.0.beta1` release, the CHANGELOGs got an entry like the
following:

```
* No changes.
```

It is kinda confusing as there are indeed changes after it. Not a
biggie, just a small pass over the CHANGELOGs.

[ci skip]