feat(angular.extend) - Simple non-breaking change to support deep extend

People use `angular.extend()` as a utility function, whether or not it's designed for that purpose. It can easily be extended to support recursive merging with very very minor additions. Developers coming to Angular expect `extend()` to support this.
If the value is `null` or `undefined`, Object.keys() will throw here... Don't worry about checking the type I guess, it's fine