When using the aiohttp example from the examples folder and use Skipper as reverse proxy Chrome logs WebSocket connection to 'ws://localhost:8000/socket.io/?EIO=3&transport=websocket' failed: Invalid frame header' failed: Invalid frame header to the console.

The aiohttp example works with nginx and without any proxy.
The asgi example works with skipper, nginx and without any proxy.
A nodejs test server also works with skipper, nginx and without any proxy.
I am using transports: ['websocket'] to force the client to use websockets only

Versions used:

• pytohn 3.7.3 (also tested with paython 3.6.8)
• aiohttp 3.5.4
• python-engineio 3.5.2
• python-socketio 4.0.2
• skipper v0.10.216

How to reproduce:

• edit app.html to force websockets only. change line 10:

var socket = io.connect('http://' + document.domain + ':' + location.port + namespace, {
    transports: ['websocket']
});

• run skipper with docker:

docker run --rm -it --network host registry.opensource.zalan.do/pathfinder/skipper:v0.10.216 skipper -experimental-upgrade -experimental-upgrade-audit -application-log-level DEBUG -address :8000 -inline-routes 'r: * -> "http://127.0.0.1:8080/"'

• run python app.py
• check console output from skipper (has websocket audit enabled) and chrome console for "invalid frame header" -> ws connection is dropped immediately after it has been established.