[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.

By: GJ <gjmcferrin@gmail.com>

paper-server/src/main/java/org/bukkit/craftbukkit/inventory/CraftInventoryView.java

@@ -82,6 +82,8 @@ public class CraftInventoryView extends InventoryView {
                     type = SlotType.RESULT;
                 } else if(slot == 1) {
                     type = SlotType.FUEL;
+                } else {
+                    type = SlotType.CRAFTING;
                 }
                 break;
             case BREWING: