RAM savings with http responseCodeStrings to progmem

As I found a lot of strings in the webserver library, I tried to patch it to use progmem with F("") and returning a string. Is seems to work fine and saves 688 bytes of global RAM.

Just this code was changed:

String ESP8266WebServer::_responseCodeToString(int code) {
  switch (code) {
    case 100: return F("Continue");
    case 101: return F("Switching Protocols");
    case 200: return F("OK");
    case 201: return F("Created");
..
..

Don't know if this is the best approach, but would like to save RAM where possible.