Clear url helpers when reloading routes

actionpack/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Clear url helpers when reloading routes.
+
+    *Santiago Pastorino*
+
 *   `BestStandardsSupport` middleware now appends it's `X-UA-Compatible` value to app's
     returned value if any. Fix #8086
 

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -288,6 +288,7 @@ def finalize!
 
       def clear!
         @finalized = false
+        @url_helpers = nil
         named_routes.clear
         set.clear
         formatter.clear