ZF\Hal\Metadata\Metadata::getHydrator returns ExtractionInterface|null instead of HydratorInterface|null

[boesing]

Hey there,
currently using the ZF\Hal\Metadata\Metadata to grab the hydrator defined in module config related to a specific entity.
As I wanted to inject the hydrator to my Zend\Stdlib\Hydrator\HydratorAwareInterface which requires the Zend\Stdlib\Hydrator\HydratorInterface, I get some IDE warnings which is quite annoying.

Since the Zend\Stdlib\Hydrator\HydratorInterface implements the Zend\Stdlib\Extractor\ExtractionInterface, there should be no problem to switch to the correct Interface.

If there is any problem, you should rename the method as of #128 since this naming is quite confusing.