Currently, RESTBase does not handle HEAD requests at all - it just throws a 404 as no route has got a HEAD handler defined. For an incoming HEAD request, we need to:

• find either a HEAD or a GET handler for the route, error out otherwise
• complete the request, and, if the GET handler was used truncate the body and remove the Content-Length header

Additionally, we need to truncate the body and remove the Content-Length header for all outgoing responses (even errors) for HEAD requests.