新增于: v12.9.0
如果调用了 outgoingMessage.end()，则为 true。
该属性不表示数据是否被刷新。
为此目的，则改用 message.writableFinished。
Added in: v12.9.0
Is true if outgoingMessage.end() has been called. This property does
not indicate whether the data has been flushed. For that purpose, use
message.writableFinished instead.