Make websocket server tolerant of HTTP/2
It causes a panic in the log when you connect to the server with e.g. curl:
2019/01/18 18:56:29 http2: panic serving X.X.X.X:YYYY: interface conversion: *http2.responseWriter is not http.Hijacker: missing method Hijack
I'm not sure if WebSocket connections from browsers use HTTP/2 yet or not, but in any case we should handle it gracefully.