Add test case for make sure SentNotification save a correct noteable value;

spec/services/notification_service_spec.rb

@@ -52,6 +52,9 @@ describe NotificationService, services: true do
         it do
           add_users_with_subscription(note.project, issue)
 
+          # Ensure create SentNotification by noteable = issue 6 times, not noteable = note
+          expect(SentNotification).to receive(:record).with(issue, any_args).exactly(6).times
+
           ActionMailer::Base.deliveries.clear
 
           notification.new_note(note)