Update StrategyInterface.php

This PR is to implement functionality as detailed in the AbstractHydrator. This should *not* be a BC break.

https://github.com/zendframework/zf2/blob/master/library/Zend/Stdlib/Hydrator/AbstractHydrator.php#L114
https://github.com/zendframework/zf2/blob/master/library/Zend/Stdlib/Hydrator/AbstractHydrator.php#L131