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.
```js
if (isDeep) --argsLength;
```

We don't need to delete anything, it's slow, and deleting the argument doesn't really do anything meaningful (it doesn't change the length value)