Feature/add headers

[derpflanz]

This adds the possibility to add maximum 3 extra header fields to the response. This is useful when using the ArduinoHttpServer to serve REST calls and you want the caller to reside somewhere else (like a local HTML file).

Usage:

httpReply.addHeader("Access-Control-Allow-Origin", "*");

Tested and works on the Arduino MKR Wifi

[replaysMike]

bump this would be great to have merged in. Lacking header support is an important oversight of the library