Do not send keep-alive in 101 (Switching Protocols) responses (#709)

Do not send keep-alive in 101 (Switching Protocols) responses (#709)

... because it breaks clients using websocket_client[1] library and is
redundant in our HTTP/1.1 control messages anyway.

I suspect that at least some buggy clients are confused by a multi-value
Connection field rather than the redundant keep-alive signal itself, but
let's try to follow RFC 7230 Upgrade example more closely this time and
send no keep-alive at all.

[1] https://pypi.org/project/websocket_client/