code: public static Response unauthorized(String entity, String challenge)
      {
          return builder(Status.UNAUTHORIZED, entity, MEDIA_TYPE_JSON)
                  .header(HttpHeaders.WWW_AUTHENTICATE, challenge)
                  .build();
      }
docs: Create a response of {@code "401 Unauthorized"}
  with the given entity formatted in
  {@code "application/json;charset=UTF-8"} and
  with {@code WWW-Authenticate} header having the
  value specified by {@code challenge}.