Allow Host to be overridden in handshake headers

[gordonread]

I had a situation with a thirdparty websocket server which was rejecting the upgrade to a websocket connection (in IXWebSocketHandshake::clientHandshake(...)) with a HTTP 400 (Bad Request) error because the thirdparty server didn't like the format of the "Host:" header being set to "stream.example.com:port", so, e.g.

GET /required/path HTTP:/1.1
Host: stream.example.com:1234

Failed with a HTTP 400 error code (when IXWebSocket expects a HTTP 101 Switching Protocols code), but the following without ":port":

GET /required/path HTTP:/1.1
Host: stream.example.com

Worked OK.

This small change allows the "Host" header to be specified in `ix::WebSocketHttpHeaders", e.g.

ix::WebSocket webSocket{};
ix::WebSocketHttpHeaders headers{};
headers["Host"] = "stream.example.com";
webSocket.SetExtraHeaders(headers);

Will give the desired result.

Added a few test cases.

[bsergean]

Thanks !