Angular testing: testing angular Services with private methods

lets say I got this kind of method on a service:

this.search = function (term) {

            var deferred = $q.defer();

             Restangular.all('search').getList(params)
                .then(_onRestResult.bind(this, deferred))
                .catch(_onRestError.bind(this, deferred));


            return deferred.promise;
        }

function _onRestResult(deferred, data) { // notice it isn't on the this

//doSomthing
}

But running a basic test.

when I change the then clause to an anonymous function, everything works as expected, but when I used the named private function I get: karma prints:

TypeError: 'undefined' is not a function (evaluating '_onRestResult.bind(this,deferred)')

I'm aware of this kind of answers

but they refer to controllers and not services (:)) and suggest not using private methods, but we really prefer using them.

I also run into this answer that suggests the private methods are implicitly tested, which was what I thought, until I run into this error messages.

Thanks for the help!

EDIT: I should notice I'm mocking Restangular like this (if this is relevant):

mockRestangular = {

                one:function(){
                    return this;
                },

                getList:function(calls){
                       answer ={results:['1','2']}

                    var deferred = $q.defer();
                    deferred.resolve(answer);
                    return deferred.promise;
                },

                post:function(called){
                    answer = 'posted: '+called;
                    var deferred = $q.defer();
                    deferred.resolve(answer);
                    return deferred.promise;

                },
                get: function(called){
                    answer = this;
                    var deferred = $q.defer();
                    deferred.resolve(answer);
                    return deferred.promise;
                },


                all:function(){
                    return this;
                }

            };