Giving slowness to rideable entity doesn't apply the slowness when item_controlled

Release_566145

Summary:
If you give an entity with rideable slowness, and then ride the entity, the slowness doesn't apply.

Impact:
We are currently working on a map that uses this as a core feature.

Retro Steps

1. Spawn pig and give it a saddle
2. Give that entity slowness
3. Mount entity and try to ride it.

Observed Behavior:
When you mount the entity it runs at full speed.

Expected Behavior:
The entity should be slowed down until the duration of the effect expires

*Screenshot/Video Attached:
https://youtu.be/u4d0uUcqXzE