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.
we just copied everything from `src` into `dst[key]` --- why are we clobbering `dst[key]` now? Should only do this if `src` is a non-object