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.
ngClassWatchAction, when called as a $watch function, gets the wrong old
value after it has been invoked previously due to observation of the
interpolated class attribute. As a result it doesn't remove classes
properly. Keeping track of the old value manually seems to fix this.
Closes#1637