Creepers cannot be lit by Dispensers

Creepers can be lit by Flint and Steels if they are held in the player's hand. If a creeper is placed in front of a dispenser with a Flint and Steel in it, it should function the same way, but it does not.

To Reproduce:
1. Spawn a creeper
2. Place a dispenser next to the creeper
3. Put a Flint and Steel into the dispenser
4. Power the dispenser. The creeper will be lit on fire, but won't explode.

Expected Behavior:
The Creeper will light and explode, as it does when lit by hand.