response.statusCode
使用隐式标头（不显式调用 response.writeHead()）时，此属性控制在标头刷新时将发送到客户端的状态码。
¥When using implicit headers (not calling response.writeHead() explicitly),
this property controls the status code that will be sent to the client when
the headers get flushed.
response.statusCode = 404; 响应头发送到客户端后，该属性表示发送出去的状态码。
¥After response header was sent to the client, this property indicates the status code which was sent out.