this method should always have a parameter passed to it, so remove the default

Author: tenderlove

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -641,7 +641,7 @@ def optimize_routes_generation?
         !mounted? && default_url_options.empty?
       end
 
-      def _generate_prefix(options = {})
+      def _generate_prefix(options)
         nil
       end