事件：'upgrade'

¥Event: 'upgrade'

• request <http.IncomingMessage> HTTP 请求的参数，如它在 'request' 事件中

  ¥request <http.IncomingMessage> Arguments for the HTTP request, as it is in the 'request' event

• socket <stream.Duplex> 服务器和客户端之间的网络套接字

  ¥socket <stream.Duplex> Network socket between the server and client

• head <Buffer> 升级流的第一个数据包（可能为空）

  ¥head <Buffer> The first packet of the upgraded stream (may be empty)

每次客户端请求 HTTP 升级时触发。监听此事件是可选的，客户端不能坚持协议更改。

¥Emitted each time a client requests an HTTP upgrade. Listening to this event is optional and clients cannot insist on a protocol change.

触发此事件后，请求的套接字将没有 'data' 事件监听器，这意味着需要绑定它才能处理发送到该套接字上的服务器的数据。

¥After this event is emitted, the request's socket will not have a 'data' event listener, meaning it will need to be bound in order to handle data sent to the server on that socket.

除非用户指定 <net.Socket> 以外的套接字类型，否则此事件保证传入 <net.Socket> 类（<stream.Duplex> 的子类）的实例。

¥This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.