Add back old constant alias for Rodauth::Rails::Mailer

This ensures that any email deliveries enqueued on earlier versions of
rodauth-rails would still get processed after deploying code with new version.

Author: janko

lib/rodauth/rails/feature/email.rb

@@ -24,6 +24,10 @@ def create_email_to(to, subject, body)
         def send_email(email)
           email.deliver_now
         end
+
+        # for backwards compatibility
+        Mailer = Rodauth::Rails::Mailer
+        deprecate_constant :Mailer
       end
     end
   end