Fix ActionMailer assertion not working for mail defining delivery_job:

- 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.

1 files changed, 5 insertions, 0 deletions

diff --git a/actionmailer/CHANGELOG.md b/actionmailer/CHANGELOG.md
index 35574a2c73..ec6f249974 100644
--- a/actionmailer/CHANGELOG.md
+++ b/actionmailer/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Fix ActionMailer assertions not working when a Mail defines
+    a custom delivery job class
+
+    *Edouard Chin*
+
 *   Mails with multipart `format` blocks with implicit render now also check for
     a template name in options hash instead of only using the action name.