Feature Request: Enable half-closed with CloseWrite

I'm writing a TCP reverse proxy over Websocket and I'm trying to use this library.

However, it provides no way to close the write side only. That's needed to indicate end of stream (EOF), which gets mapped to a FIN on the TCP connection to the origin.

Looking at the code, I see that Close() runs the full handhsake. Perhaps CloseWrite could just send the close message with the normal status, but not wait for the response. And the code should not close the connection when receiving the close message. It should only do that on the explicit Close() or CloseWrite() call by the application code.

首先跟踪现有的 Close() 握手流程以及处理接收到的关闭消息的代码。定义显式 CloseWrite() 应如何在不等待响应或关闭连接的情况下发出写入侧 EOF 信号，同时保留通过 Close() 完全关闭的能力；通过针对性的 WebSocket 和 TCP-proxy 测试验证行为。

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