SetDeadline should only close the connection if there is an in progress Read/Write

I'm using `websocket.NetConn`. `net.Conn` docs say:

>A deadline is an absolute time after which I/O operations fail with a timeout (see type Error) instead of blocking.

However this closes the connection 10s after the write, even if the write has already completed:

```
conn.SetWriteDeadline(time.Now().Add(10 * time.Second)
conn.Write([]byte("hello world"))
```