Close internal Pipes in websocketconn.Conn Close.

Unless something externally called Write after Close, the
writeLoop(ws, pr2) goroutine would run forever, because nothing would
ever close pw2/pr2.
https://bugs.torproject.org/33367#comment:4

common/websocketconn/websocketconn.go

@@ -24,6 +24,8 @@ func (conn *Conn) Write(b []byte) (n int, err error) {
 }
 
 func (conn *Conn) Close() error {
+	conn.Reader.(*io.PipeReader).Close()
+	conn.Writer.(*io.PipeWriter).Close()
 	// Ignore any error in trying to write a Close frame.
 	_ = conn.Conn.WriteControl(websocket.CloseMessage, []byte{}, time.Now().Add(time.Second))
 	return conn.Conn.Close()