fix(angular.equals): consistently compare undefined object props

previously:

a = {};
b = {x:undefined};
angular.equals(a, b) == angular.equals(b, a) // returns false.

both should return false because these objects are not equal.

unlike in implemented in #1695 the comparison should be stricter
and return false not true. if we need to relax this in the future
we can always do so.

Closes #1648