Do not require passing :app to mounted helpers, it's actually useless and not DRY

actionmailer/lib/action_mailer/railtie.rb

@@ -21,7 +21,7 @@ module ActionMailer
       ActiveSupport.on_load(:action_mailer) do
         include AbstractController::UrlFor
         extend ::AbstractController::Railties::RoutesHelpers.with(app.routes)
-        include app.routes.mounted_helpers(:app)
+        include app.routes.mounted_helpers
         options.each { |k,v| send("#{k}=", v) }
       end
     end

actionpack/lib/action_controller/railtie.rb

@@ -27,7 +27,7 @@ module ActionController
       options.page_cache_directory ||= paths.public.to_a.first
 
       ActiveSupport.on_load(:action_controller) do
-        include app.routes.mounted_helpers(:app)
+        include app.routes.mounted_helpers
         extend ::AbstractController::Railties::RoutesHelpers.with(app.routes)
         extend ::ActionController::Railties::Paths.with(app)
         options.each { |k,v| send("#{k}=", v) }

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -262,7 +262,7 @@ module ActionDispatch
       module MountedHelpers
       end
 
-      def mounted_helpers(name = nil)
+      def mounted_helpers(name = :app)
         define_mounted_helper(name) if name
         MountedHelpers
       end

actionpack/test/dispatch/prefix_generation_test.rb

@@ -72,7 +72,7 @@ module TestGenerationPrefix
 
     class ::InsideEngineGeneratingController < ActionController::Base
       include BlogEngine.routes.url_helpers
-      include RailsApplication.routes.mounted_helpers(:app)
+      include RailsApplication.routes.mounted_helpers
 
       def index
         render :text => posts_path