Angular unit test factory that inherits from $resource

I'm actually overriding the way $resource works, by extending it using the following code :

'use strict';

angular
  .module('Exo.api')
  .factory('Resource', function ($resource) {
    Resource = function (url, params, methods) {
      defaults = {
        update: { method: 'PATCH' },
        create: { method: 'POST' }
      }

      methods  = angular.extend(defaults, methods);
      resource = $resource(url, params, methods);

      resource.prototype.$save = function () {
        var action = !this.id ? '$create' : '$update';
        return this[action]();
      };

      return resource;
    }
    return Resource;
  )

However, I have a hard time to test it and to be honest, don't know where to start exactly.

What I did so far is :

'use strict';

describe('Exo.api.Resource', function () {
  Resource = {}
  User     = {}

  beforeEach(module('Exo.api'));
  beforeEach(inject(function (_Resource_) {
    Resource = _Resource_;
    User     = Resource('/users/:id', { id: '@id' })
  }));

  it('should be defined', function () {
    expect(Resource).toBeDefined();
  });

  describe('$save', function () {
    beforeEach(function () {
      spyOn(Resource, '$update');
      spyOn(Resource, '$create');
    });

    describe('when object has an id', function () {
      it('should have call the $update method', function () {
        user = new User({ id: 1, name: 'Chuck Norris' });
        user.$save()
        expect(Resource.$update).toHaveBeenCalled();
      });
    });

    describe('when object does not have an id', function () {
      it('should have call the $create method', function () {
        user = new User({ name: 'Chuck Norris' });
        user.$save()
        expect(Resource.$create).toHaveBeenCalled();
      });
    });
  });
});

Unfortunately, this is not working at all. Any help? Thanks