call default_middleware_stack before merging stacks

`default_middleware_stack` seems to kick off the `on_load` calls that
may mutate the middleware stack.  We have to call that method before
merging middleware stacks, otherwise the middleware stacks get mutated
*after* the app middleware stack is built.

railties/lib/rails/engine.rb

@@ -508,7 +508,8 @@ module Rails
     def app
       @app || @app_build_lock.synchronize {
         @app ||= begin
-          config.middleware = build_middleware.merge_into(default_middleware_stack)
+          stack = default_middleware_stack
+          config.middleware = build_middleware.merge_into(stack)
           config.middleware.build(endpoint)
         end
       }