2.x Return by Reference Functions

I assumed that the use of return by reference is legacy code, thus removed them.

By the way ``ClassRegistry::_duplicate()`` does not seem to be covered by tests explicitly.
But it is used [here](https://github.com/ravage84/cakephp/blob/b3dfad614ade26ba68c08bc52fb750dcd6875bac/lib/Cake/Utility/ClassRegistry.php#L137) only, anyway.