Bug in SortableMixin

There is a bug in the SortableMixin, there is a failing test that demonstrates it below.

This test fails as all items are removed from the array proxy, the expected result is that only one item is removed.

```.js
test("removing the last item of a sorted array proxy only removes last item", function() {
  var sortedArrayProxy = Ember.ArrayProxy.create(Ember.SortableMixin,{
    sortProperties: ['id'],
    sortAscending: false,
    content: Ember.A([{id: 1}, {id: 2}])
  });
  equal( sortedArrayProxy.get('length'), 2, 'precond - array has 2 items');
  sortedArrayProxy.removeObject(sortedArrayProxy.get('lastObject'));
  equal( sortedArrayProxy.get('length'), 1, 'array has 1 item');
});
```