Mobs will sometimes spin when walking on top of rails

The Bug:

Mobs will sometimes spin when walking on top of rails.

Steps to Reproduce:

1. Build the setup shown in the attachment below. setup.png
2. Summon a creeper on top of the rail with a diamond block below it.
3. Wait for it to begin walking and watch closely.
4. Take note as to whether or not mobs will sometimes spin when walking on top of rails.

Observed Behavior:

Mobs will sometimes spin.

Expected Behavior:

Mobs would not spin.