Support websocket over tunnel

[zllovesuki]

{
  "level": "error",
  "ts": 1661767160.5492272,
  "caller": "gateway/http_handler.go:96",
  "msg": "forwarding to client",
  "component": "gateway",
  "error": "http2: invalid Upgrade request header: [\"websocket\"]",
  "stacktrace": "kon.nect.sh/specter/tun/gateway.(*Gateway).errorHandler\n\t/home/rachel/code/specter/tun/gateway/http_handler.go:96\nnet/http/httputil.(*ReverseProxy).ServeHTTP\n\t/usr/local/go/src/net/http/httputil/reverseproxy.go:312\nnet/http.serverHandler.ServeHTTP\n\t/usr/local/go/src/net/http/server.go:2947\nnet/http.(*conn).serve\n\t/usr/local/go/src/net/http/server.go:1991"
}