Websocket header case sensitivity

[Mark Harkin]

Traefik2 chooses to use lower case 's' in 'Sec-Websocket-Accept' 'Sec-Websocket-Protocol'. This breaks xpra wss connection with traefik ssl termination. A little annoying but http header parsing should be case insensitive.

The below fix worked for me:

​https://github.com/mjharkin/Xpra/commit/772eafb0164560fa0e8f4f574f383d80bd82f245

[Antoine Martin]

Thanks!

Applied in r27480.

[Antoine Martin]

Damn.

There is no CaseInsensitiveDict in python2, so the backport broke all the python2 builds.
Fixed the lazy way (no support for case insensitive attributes with python2): r27624.

It also broke packaging on macos.

So at that point, I decided to just re-write it without using CaseInsensitiveDict: r27626.