'upgrade' 事件

• request <http.IncomingMessage> HTTP 请求的参数，如它在 'request' 事件中
• socket <stream.Duplex> 服务器和客户端之间的网络套接字
• head <Buffer> 升级流的第一个数据包（可能为空）

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。

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。

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。

• request <http.IncomingMessage> Arguments for the HTTP request, as it is in the 'request' event
• socket <stream.Duplex> Network socket between the server and client
• head <Buffer> The first packet of the upgraded stream (may be empty)

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

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.

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>.