Fixed a VERY long standing bug discovered by libraryaddict.

Constructing a WrappedWatchableObject with a 
net.minecraft.server.ItemStack would cause ProtocolLib to throw an
IllegalArgumentException, even though WrappedWatchableObject.
setValue(Object) accepts ItemStacks without trouble.

This is because WrappedWatchableObject.getUnwrappedType() didn't handle 
ItemStacks at all.