avoid another call to `scope`

calling `scope` isn't cheap, so try to call cheaper methods that do the
same thing for those particular parameters (in this case `path_scope`)

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -1591,7 +1591,7 @@ def root(path, options={})
 
           if @scope.resources?
             with_scope_level(:root) do
-              scope(parent_resource.path) do
+              path_scope(parent_resource.path) do
                 super(options)
               end
             end