Make sure that mailers will use default job queue

Use "default" as default queue for mailers test helpers

actionmailer/lib/action_mailer/test_helper.rb

@@ -123,13 +123,13 @@ module ActionMailer
     #       ContactMailer.with(email: 'user@example.com').welcome.deliver_later
     #     end
     #   end
-    def assert_enqueued_email_with(mailer, method, args: nil, queue: "mailers", &block)
+    def assert_enqueued_email_with(mailer, method, args: nil, queue: ActionMailer::Base.deliver_later_queue_name || "default", &block)
       args = if args.is_a?(Hash)
         [mailer.to_s, method.to_s, "deliver_now", params: args, args: []]
       else
         [mailer.to_s, method.to_s, "deliver_now", args: Array(args)]
       end
-      assert_enqueued_with(job: mailer.delivery_job, args: args, queue: queue, &block)
+      assert_enqueued_with(job: mailer.delivery_job, args: args, queue: queue.to_s, &block)
     end
 
     # Asserts that no emails are enqueued for later delivery.

actionmailer/test/test_helper_test.rb

@@ -34,6 +34,14 @@ end
 class TestHelperMailerTest < ActionMailer::TestCase
   include ActiveSupport::Testing::Stream
 
+  setup do
+    @previous_deliver_later_queue_name = ActionMailer::Base.deliver_later_queue_name
+  end
+
+  teardown do
+    ActionMailer::Base.deliver_later_queue_name = @previous_delivery_method
+  end
+
   def test_setup_sets_right_action_mailer_options
     assert_equal :test, ActionMailer::Base.delivery_method
     assert ActionMailer::Base.perform_deliveries
@@ -317,6 +325,54 @@ class TestHelperMailerTest < ActionMailer::TestCase
     end
   end
 
+  def test_assert_enqueued_email_with_when_deliver_later_queue_name_is_nil
+    ActionMailer::Base.deliver_later_queue_name = nil
+
+    assert_nothing_raised do
+      assert_enqueued_email_with TestHelperMailer, :test do
+        silence_stream($stdout) do
+          TestHelperMailer.test.deliver_later
+        end
+      end
+    end
+  end
+
+  def test_assert_enqueued_email_with_when_deliver_later_queue_name_with_non_default_name
+    ActionMailer::Base.deliver_later_queue_name = "sample_mailer_queue_name"
+
+    assert_nothing_raised do
+      assert_enqueued_email_with TestHelperMailer, :test do
+        silence_stream($stdout) do
+          TestHelperMailer.test.deliver_later
+        end
+      end
+    end
+  end
+
+  def test_assert_enqueued_email_with_when_deliver_later_queue_name_is_symbol
+    ActionMailer::Base.deliver_later_queue_name = :mailers
+
+    assert_nothing_raised do
+      assert_enqueued_email_with TestHelperMailer, :test do
+        silence_stream($stdout) do
+          TestHelperMailer.test.deliver_later
+        end
+      end
+    end
+  end
+
+  def test_assert_enqueued_email_with_when_queue_arg_is_symbol
+    ActionMailer::Base.deliver_later_queue_name = "mailers"
+
+    assert_nothing_raised do
+      assert_enqueued_email_with TestHelperMailer, :test, queue: :mailers do
+        silence_stream($stdout) do
+          TestHelperMailer.test.deliver_later
+        end
+      end
+    end
+  end
+
   def test_assert_enqueued_email_with_when_mailer_has_custom_delivery_job
     assert_nothing_raised do
       assert_enqueued_email_with CustomDeliveryMailer, :test do