Cache url_helpers instead of creating each time

This has 2 effects:

1. RoutesProxy is CRAZY faster because it's no longer creating a new
Module each time method_missing is hit.
2. It bypasses an existing bug in ruby that makes `class << obj` unsafe
to be used in threading contexts.

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -395,9 +395,11 @@ def define_mounted_helper(name)
         return if MountedHelpers.method_defined?(name)
 
         routes = self
+        helpers = routes.url_helpers
+
         MountedHelpers.class_eval do
           define_method "_#{name}" do
-            RoutesProxy.new(routes, _routes_context)
+            RoutesProxy.new(routes, _routes_context, helpers)
           end
         end
 

actionpack/lib/action_dispatch/routing/routes_proxy.rb

@@ -8,8 +8,9 @@ class RoutesProxy #:nodoc:
       attr_accessor :scope, :routes
       alias :_routes :routes
 
-      def initialize(routes, scope)
+      def initialize(routes, scope, helpers=nil)
         @routes, @scope = routes, scope
+        @helpers = helpers || routes.url_helpers
       end
 
       def url_options
@@ -19,16 +20,16 @@ def url_options
       end
 
       def respond_to?(method, include_private = false)
-        super || routes.url_helpers.respond_to?(method)
+        super || @helpers.respond_to?(method)
       end
 
       def method_missing(method, *args)
-        if routes.url_helpers.respond_to?(method)
+        if @helpers.respond_to?(method)
           self.class.class_eval <<-RUBY, __FILE__, __LINE__ + 1
             def #{method}(*args)
               options = args.extract_options!
               args << url_options.merge((options || {}).symbolize_keys)
-              routes.url_helpers.#{method}(*args)
+              @helpers.#{method}(*args)
             end
           RUBY
           send(method, *args)