No experience orbs when furnace empties into hopper

Put the summary of the bug you're having here

What I expected to happen was...:
When a furnace empties into a hopper, exp orbs should come out, as if you removed the smelted items from the furnace by hand.

What actually happened was...:
When a furnace empties into a hopper, no exp comes out

Steps to Reproduce:
1. place furnace on top of hopper
2. smelt ore in furnace
3. watch smelted items trickle into hopper, releasing no experience orbs