http2 response writer panic & short write errors

Experiencing errors in running latest cloudflared tunnel (server-side, proxying web server tcp, used for long-polling&long-download), but no errors are thrown on cloudflared client side.

Web server is running pure http/1.1, no http/2 support. Web server is encapsulated with tcp (tcp://127.0.0.1:8081), not directly http (cannot be access from browser, must use cloudflared client)

```
2021-08-30T18:00:23Z INF Connection b283f476-5c89-4578-992c-cc10f12dcecd registered connIndex=0 location=DUS
2021-08-30T18:00:24Z INF Connection c6ea521a-59b5-420b-b49f-1596d5b8a545 registered connIndex=1 location=DUS
2021-08-30T18:00:25Z INF Connection e04925fb-b9be-4c1d-8d58-b2d7713099e7 registered connIndex=2 location=AMS
2021-08-30T18:00:26Z INF Connection 2d6693ed-ccce-4752-84b5-e414af665fc8 registered connIndex=3 location=AMS
Recover from http2 response writer panic, error (0xf29980,0x124f840)
2021-08-30T18:04:25Z ERR failed to write ping message error="short write"
2021-08-30T18:05:23Z ERR failed to write ping message error="short write"
2021-08-30T18:05:29Z ERR failed to write ping message error="short write"
```

Do you know why that happens? Does it have something to do with my connection? I saw other person having similar issue #411, but I'm not using docker/service, it's running on pure terminal, ubuntu 20.04.

From #411 it's says cloudflared -> tcp service issue, but looking at code, it seems to be connection between cloudflare edge and the cloudflared.