Steps to Reproduce:

1. Get an undead mob's spawn egg.

2. Spawn it on any block that could prevent it from burning on day time (full block, slab...).

3. Spawn it under any type of stair (no matter its rotation).

Observed Results:
When trying to protect an undead mob with a stair it still burns on day time.

Expected Results:
Having a stair block upwards an undead mob should prevent it from catching fire of the day.