[Bleeding] Return correct furnace SlotType. Fixes BUKKIT-5400

Previously, the SlotType for the 0 slot in a furnace returned CONTAINER,
when it should have returned SlotType.CRAFTING. This updates the code for
determining slot type to return the proper value.

(cherry picked from commit abf90f945408fed2274321b2f9133f8f9a89c21c)