QHttpServer: returning a value from a route handler should be an error when the function uses a QHttpServerResponder

A handler for a route can either return a value from the function which will be used as a response, e.g.

[](...) {
  return "Hello";
}

Or it can accept the special QHttpServerResponder object as a parameter.

[](QHttpServerResponder &&responder) {
  responder.write("hello");
}

When someone does both, having the QHttpServerResponder argument, as well as trying to return a string, then the return doesn't do anything.

In fact it should probably be an error to return anything other than void in this case.

[](**QHttpServerResponder &&responder**) {
  **return** "Hello"; // It would be good if this was a compile time error.
}