Alias app to build_middleware_stack for clarity.

railties/lib/rails/application.rb

@@ -160,6 +160,7 @@ module Rails
         config.middleware.build(routes)
       end
     end
+    alias :build_middleware_stack :app
 
     def call(env)
       app.call(env.reverse_merge!(env_defaults))

railties/lib/rails/application/finisher.rb

@@ -32,7 +32,7 @@ module Rails
       end
 
       initializer :build_middleware_stack do
-        app
+        build_middleware_stack
       end
 
       initializer :eager_load! do