Cache url_helpers

`url_helpers` used to be memoized. This was lost in a refactoring and
this PR adds it back. We noticed this while investigating why
integration tests are slower than controller tests.

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -410,6 +410,7 @@ module ActionDispatch
       end
 
       def url_helpers(supports_path = true)
+        @url_helpers ||= begin
           routes = self
 
           Module.new do
@@ -467,6 +468,7 @@ module ActionDispatch
             private :_generate_paths_by_default
           end
         end
+      end
 
       def empty?
         routes.empty?