RESTLET 2.1RC1 : ChallengeResponse send from GWT client to server always NULL
ClientResource r = new ClientResource("/service" + UsersServiceProxy.URL);
    r.setChallengeRespon​se(ChallengeScheme.H​TTP_BASIC, "user1", "secret");
    r.setOnResponse(new Uniform() {
        public void handle(Request request, Response response) {
           .....
        }
    });
    r.get();

UserServiceProxy userService = GWT.create(UserServiceProxy.class);
    userService.getClientResource().setReference(url);
    userService.getClientResource().setChallengeResponse(new ChallengeResponse(ChallengeScheme.HTTP_BASIC, "user1", "secret"));
    userService.getUser(username + ":" + password, new Result<User>() {

        @Override
        public void onFailure(Throwable caught) {
                ......
        }

        @Override
        public void onSuccess(User user) {
            .....
        }
    });