Make DoctrineObject use AbstractHydrator's namingStrategy

Currently the `DoctrineObject` hydrator doesn't use `AbstractHydrator::$namingStrategy`, which means we can't use `underscore_fields` in forms when our entity uses camelCase as field names. This commit fixes that. I haven't written any tests since I wouldn't be able match the quality of the other tests in this repo. All existing test do work, of course.