Slack's WebSocket API requires lowercase "websocket" in the Upgrade header

[afresh1]

As described in a ticket for the Python Slack RTM client, although RFC 6455 says:

3. An |Upgrade| header field containing the value "websocket",
   treated as an ASCII case-insensitive value.

Slack has decided to be more particular. I don't know whether we should adjust this, but the RFC does use lowercase "websocket" in its examples, so it seems likely that other folks will get this wrong.

[vti]

Maybe making it configurable is an option. It's unfortunate that because of some "big tech" decisions or bugs we have to change totally valid code.

[afresh1]

It is incredibly annoying. I would much prefer they just fixed it.