Hash::nest() should throw an exception instead of returning an empty arr...

...ay

Refs: https://github.com/cakephp/cakephp/pull/3498#issuecomment-48316204
@jippi What would be better?

@dereuromark because I simply mimicked existing behavior:
https://github.com/ravage84/cakephp/blob/hash-nest-exception/lib/Cake/Utility/Hash.php#L53