事件：'upgrade'

🌐 Event: 'upgrade'

• request <http.IncomingMessage> HTTP 请求的参数，就像在 'request' 事件中一样
• socket <stream.Duplex> 服务器和客户端之间的网络套接字
• head <Buffer> 升级后的流的第一个数据包（可能为空）

每当客户端请求 HTTP 升级时触发。监听此事件是可选的，客户端不能要求更改协议。

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

在此事件被触发后，请求的套接字将不会有 'data' 事件监听器，这意味着需要绑定监听器才能处理发送到该套接字的服务器数据。

🌐 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.

此事件保证会传递一个 <net.Socket> 类的实例，该类是 <stream.Duplex> 的子类，除非用户指定了其他类型的套接字而不是 <net.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>.