Wrap routes.url_helpers.url_for via a proxy

The singleton url_for on Rails.application.routes.url_helpers isn't the
same as the url_for you get when you include the module in your class as
the latter has support for polymorphic style routes, etc. whereas the
former accepts only a hash and is the underlying implementation defined
on ActionDispatch::Routing::RouteSet.

This commit changes the singleton method to call through a proxy instance
so that it gets the full range of features specified in the documentation
for url_for.

Author: pixeltrix

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -446,17 +446,34 @@ def url_helpers(supports_path = true)
 
           # Define url_for in the singleton level so one can do:
           # Rails.application.routes.url_helpers.url_for(args)
-          @_routes = routes
+          proxy_class = Class.new do
+            include UrlFor
+            include routes.named_routes.path_helpers_module
+            include routes.named_routes.url_helpers_module
+
+            attr_reader :_routes
+
+            def initialize(routes)
+              @_routes = routes
+            end
+
+            def optimize_routes_generation?
+              @_routes.optimize_routes_generation?
+            end
+          end
+
+          @_proxy = proxy_class.new(routes)
+
           class << self
             def url_for(options)
-              @_routes.url_for(options)
+              @_proxy.url_for(options)
             end
 
             def optimize_routes_generation?
-              @_routes.optimize_routes_generation?
+              @_proxy.optimize_routes_generation?
             end
 
-            attr_reader :_routes
+            def _routes; @_proxy._routes; end
             def url_options; {}; end
           end