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

1 files changed, 13 insertions, 0 deletions

diff --git a/actionmailer/test/message_delivery_test.rb b/actionmailer/test/message_delivery_test.rb
index c0683be94d..51f10b0bf1 100644
--- a/actionmailer/test/message_delivery_test.rb
+++ b/actionmailer/test/message_delivery_test.rb
@@ -95,6 +95,19 @@ class MessageDeliveryTest < ActiveSupport::TestCase
     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)