Wither roses are made even though mobGriefing is false

The bug

When a wither kills a mob, it creates a wither rose on the place where it died.
It shouldn't do that when mobGriefing is set to false, but it still does.

How to reproduce

1. Set mobGriefing to false

   /gamerule mobGriefing false

2. Spawn a wither and a pig
   → The wither will kill the pig, and place a wither rose where it died.