fix(ngClass): should remove classes when object is the same but property has changed

If you wire up ngClass directly to an object on the scope, e.g. ng-class="myClasses",
where scope.myClasses = { 'classA': true, 'classB': false },
there was a bug that changing scope.myClasses.classA = false, was not being picked
up and classA was not being removed from the element's CSS classes.

This fix uses angular.equals for the comparison and ensures that oldVal is a copy of
(rather than a reference to) the newVal.