mounted helpers should be included in ActionMailer

actionmailer/lib/action_mailer/railtie.rb

@@ -19,8 +19,9 @@ module ActionMailer
 
       ActiveSupport.on_load(:action_mailer) do
         include app.routes.url_helpers
+        include app.routes.mounted_helpers(:app)
         options.each { |k,v| send("#{k}=", v) }
       end
     end
   end
-end
+end

railties/test/application/initializers/frameworks_test.rb

@@ -61,6 +61,7 @@ module ApplicationTests
 
       require "#{app_path}/config/environment"
       assert Foo.method_defined?(:foo_path)
+      assert Foo.method_defined?(:app)
       assert_equal ["notify"], Foo.action_methods
     end