outgoingMessage.writableEnded
只读，如果 outgoingMessage.end() 已被调用，则为 true。
注意，这个属性不反映数据是否已经刷新。
为此目的，则改用 message.writableFinished。
Readonly, true if outgoingMessage.end() has been called. Noted that
this property does not reflect whether the data has been flush. For that
purpose, use message.writableFinished instead.