Allow mailers to configure their delivery job

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

actionmailer/CHANGELOG.md

+* Allow ActionMailer classes to configure their delivery job
+
+    class MyMailer < ApplicationMailer
+      self.delivery_job = MyCustomDeliveryJob
+
+      ...
+    end
+
+  *Matthew Mongeau*
+
 Please check [5-1-stable](https://github.com/rails/rails/blob/5-1-stable/actionmailer/CHANGELOG.md) for previous changes.

actionmailer/lib/action_mailer/base.rb

@@ -459,6 +459,7 @@ def _protected_ivars # :nodoc:
 
     helper ActionMailer::MailHelper
 
+    class_attribute :delivery_job, default: ::ActionMailer::DeliveryJob
     class_attribute :default_params, default: {
       mime_version: "1.0",
       charset:      "UTF-8",

actionmailer/lib/action_mailer/message_delivery.rb

@@ -118,7 +118,8 @@ def enqueue_delivery(delivery_method, options = {})
             "method*, or 3. use a custom Active Job instead of #deliver_later."
         else
           args = @mailer_class.name, @action.to_s, delivery_method.to_s, *@args
-          ::ActionMailer::DeliveryJob.set(options).perform_later(*args)
+          job = @mailer_class.delivery_job
+          job.set(options).perform_later(*args)
         end
       end
   end

actionmailer/test/message_delivery_test.rb

@@ -95,6 +95,19 @@ def test_should_enqueue_and_run_correctly_in_activejob
     end
   end
 
+  test "should enqueue the job with the correct delivery job" do
+    old_delivery_job = DelayedMailer.delivery_job
+    DelayedMailer.delivery_job = DummyJob
+
+    assert_performed_with(job: DummyJob, args: ["DelayedMailer", "test_message", "deliver_now", 1, 2, 3]) do
+      @mail.deliver_later
+    end
+
+    DelayedMailer.delivery_job = old_delivery_job
+  end
+
+  class DummyJob < ActionMailer::DeliveryJob; end
+
   test "can override the queue when enqueuing mail" do
     assert_performed_with(job: ActionMailer::DeliveryJob, args: ["DelayedMailer", "test_message", "deliver_now", 1, 2, 3], queue: "another_queue") do
       @mail.deliver_later(queue: :another_queue)