Merge pull request #37608 from bogdanvlviv/fix-assertions-for-mail-with-legacy-parameterized-delivery-job

Fix ActionMailer assertions don't work for parameterized mail with legacy delivery job

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

Generally followed the pattern for https://github.com/rails/rails/pull/32034

* Removes needless CI configs for 2.4
* Targets 2.5 in rubocop
* Updates existing CHANGELOG entries for fewer merge conflicts
* Removes Hash#slice extension as that's inlined on Ruby 2.5.
* Removes the need for send on define_method in MethodCallAssertions.

Add `MailDeliveryJob` for delivering both regular and parameterized mail.
Deprecate using `DeliveryJob` and `Parameterized::DeliveryJob`.

Deliver parameterized mail with `ActionMailer::DeliveryJob`
and remove `ActionMailer::Parameterized::DeliveryJob`.

- If a Mail defines a custom delivery_job, all ActionMailer assertion
  helper (assert_emails, assert_enqueued_emails ...) wouldn't work.

  ```ruby
    MyMailer < ApplicationMailer
      self.delivery_job = MyJob
    end

    # This assertion will fail
    assert_emails(1) do
      MyMailer.my_mail.deliver_later
    end

  This PR leverage the new ActiveJob feature that accepts Procs for the
  `only` keyword and check if the delivery job is one of ActionMailer
   registered ones.

Setting parameterized_delivery_job on a mailer class will cause Parameterized::MessageDelivery to use
the specified job instead of ActionMailer::Parameterized::DeliveryJob:

    class MyMailer < ApplicationMailer
      self.parameterized_delivery_job = MyCustomDeliveryJob
      ...
    end

Since these changes related to the public API, I think we should add
changelog entries.

Related to #33838, #33849

Example of `assert_enqueued_with` with no block
```ruby
def test_assert_enqueued_with
  MyJob.perform_later(1,2,3)
  assert_enqueued_with(job: MyJob, args: [1,2,3], queue: 'low')

  MyJob.set(wait_until: Date.tomorrow.noon).perform_later
  assert_enqueued_with(job: MyJob, at: Date.tomorrow.noon)
end
```

Example of `assert_enqueued_email_with` with no block:
```ruby
def test_email
  ContactMailer.welcome.deliver_later
  assert_enqueued_email_with ContactMailer, :welcome
end

def test_email_with_arguments
  ContactMailer.welcome("Hello", "Goodbye").deliver_later
  assert_enqueued_email_with ContactMailer, :welcome, args: ["Hello", "Goodbye"]
end
```

Related to #33243

* ActionMailer::Base can unregister observer(s) and interceptor(s).

One or multiple mail observers can be unregistered using
`ActionMailer::Base.unregister_observers` or
`ActionMailer::Base.unregister_observer`.

One or multiple mail interceptors can be unregistered using
`ActionMailer::Base.unregister_interceptors` or
`ActionMailer::Base.unregister_interceptor`.

For preview interceptors, it's possible to use
`ActionMailer::Base.unregister_preview_interceptors` or
`ActionMailer::Base.unregister_preview_interceptor`.

* Ensure to be reset registered observer(s) and interceptor(s)

* Add explanation to CHANGELOG

* Add original author's name

[Kota Miyake + Rafael Mendonça França + Claudio Ortolina]

* 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]

Skipping over 2.4.0 to sidestep the `"symbol_from_string".to_sym.dup` bug.

References #32028

:tada::tada::tada:

* 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]