Changed creature checks to support sub-classes in other mods

Mods that add creatures sub-classes can be problematic because the check for them in the existing code will evaluate to False because the class isn't exactly the same. This is particularly problematic for the SpecialMobs mod because the custom Wither Skeletons will not be recognized as skeletons and so Necrotic Bones will never be triggered to drop. This patch fixes that problem.