Furnaces storing XP don't reset XP when collected by player

This is marked as Private due to my considering it an XP exploit related to improper mechanics of smelting.

When setting up a furnace (or its related blocks) with hoppers in and out, the furnace stores XP until broken or a user removes an item from the furnace, as intended.

But when a player uses a switch to prevent the hopper from pulling the item out, they can continuously receive the aggregate experience many times - it does not reset after the item is removed by hand - and quickly gain hundreds of levels.

Steps to Reproduce:

1. Place a hopper feeding into a furnace and a hopper pulling from the furnace, and a switch on the side.
2. Smelt a few stacks of ore.
3. When there are a handful of items left in the furnace, take note of your XP level, flip the switch, and remove an item.
4. Upon exiting the furnace UI, note your XP level again. Re-enter the furnace UI and pull another item out.

Observed Result:

The player receives the full amount of experience they received in step #3, plus the experience from the new item in step #4.

Expected Result:

**Removing the item in step #3 should give XP as it does, and subsequently reset the stored XP count to 0. Removing the item in step #4 should only give experience gained from items smelted in the interim (since the previous removal).