Chunk function should return the value of closure

When using the chunk closure, we should still be able to get the value of the return in the closure.

     $return = Products::chunk(10, function($products) { return false; });
     var_dump($return); // (bool) false
this do look more logical with `do { } while(count($results) > 0);`, but not sure @taylorotwell is okay with that.