Sort plugin evaluates insert index regardless of change

[akagomez]

var list = new can.List([
  { firstName: 'Chris', lastName: 'Gomez' },
  { firstName: 'Justin', lastName: 'Meyer' }
]);

list.attr('comparator', 'firstName'); 
list.attr('0.lastName', 'First'); // _getInsertIndex is called needlessly

list.attr('comparator', function (a, b) {
  a = a.attr('firstName');
  b = b.attr('firstName');
  return a === b ? 0 : a > b ? -1 : 1; // Desc
}); 
list.attr('1.lastName', 'Last'); // _getInsertIndex is called needlessly