[api doc test] node-http-proxy now emits `websocket:*` on important WebSocket events. Added tests for these features and updated some code docs

[fix] Change sec-websocket-location header when proxying WSS --> WS. Added test coverage for this scenario

[fix] Set `x-forwarded-for` from req.connection.socket.remoteAddress if req.connection.remoteAddress is not defined

[api minor] Small refactor to emit `webSocketProxyError` from a single helper function on any of the various `error` events in the proxy chain

[fix doc] Add `error` handler to reverseProxy request when proxying WebSockets to prevent unhandled ParseError. Rename some variables in proxyWebSocketRequest to make the code more readable

[doc test api] Improve node-http-proxy API to allow for HTTPS to HTTP proxying scenarios. Update tests accordingly.