Initialize @_routes if it doesn't exists.

actionpack/lib/action_dispatch/routing/url_for.rb

@@ -128,6 +128,7 @@ def url_for(options = nil)
         when String
           options
         when nil, Hash
+          @_routes ||= nil
           _routes.url_for((options || {}).reverse_merge!(url_options).symbolize_keys)
         else
           polymorphic_url(options)