Wither does not reduce the light level

Summary:
The Wither does not reduce the internal light level to 11, causing zombies, zombie villagers, drowned, skeletons, strays, and phantoms to burn when exposed to the sun.

Steps to Reproduce:

1. Spawn a wither during the day
2. Spawn a mob that burns in sunlight

Observed Results:
The Wither does not reduce the light level to 11, causing the mob to burn.

Expected Results:
The Wither should reduce the light level to 11.