[AI] Mob / Creature AI will not function when standing on mob spawners.

[Description]

Mob / Creature AI will not function when standing on mob spawners.

[Reproduction Steps]

- Start a new game in creative mode on a flat world.

• Acquire a mob spawner, a spawn vindicator & a spawn villager eggs respectively, and something to wall them in, like wooden planks.
• Place the mobster spawners down and surround them with walls at least 2 blocks high.
• Spawn a villager and a vindicator.

[Result]

Mob AI does not function when on a mob spawner, the mobs will not be able to move, react to each other or attack each other. This is present for all ground based mobs. 

[Build]

Public Version 1.8.1