Expose more ways to obtain information about requests

[Mis1eader-dev]

Is your feature request related to a problem? Please describe.
In a reverse proxy application it is essential to know information about a request like its protocol (http, ws, etc.), or whether the request itself is a WebSocket.

Otherwise drogon::app().forward(...) can't make proper forwarding to the backend without rewriting what is already implemented in HttpServer.cc.

Describe the solution you'd like
Several options:

1. Expose the protocol string.
2. Add a local boolean member within requests called isWebSocket.
3. Expose the already implemented method in HttpServer.cc.

Describe alternatives you've considered
Rewriting the current implementation.