/ Effect (speed) command does not work properly when using it on some mob

^{Update by [Mojang] Mega_Spud (Jay Wells)}

The speed effect doesn't apply to all mobs/entities.

What I expected to happen:
I expected the command /effect @e speed 150 50 to apply to all entity/mobs

What actually happened:
Several mobs were unaffected.

Steps to Reproduce:

• Use the command /effect @e speed 150 50
• Notice that some mobs are not affected

List of mobs that don't appear to get the speed affect (update):

• Creeper
• Villager
• Zombie
• Pigman
• Sheep

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Original Description:
Steps to Reproduce

1.Use the / effect (Speed) command on some mob for eg a zombie. As you can see the zombie still gets its normal speed instead of increasing it