Slime/magma cubes not affected by jump boost potion effect

The bug

Slimes and magma cubes do not jump higher with the Jump Boost effect, although it still reduces fall damage for Slimes.

Steps to reproduce

1. Spawn Slime
2. Run command

   /effect give @e[type=slime] minecraft:jump_boost 60 10 false

Now you should see the particle effect, but the slime doesn't jump higher than normal.

For example see attached video or http://youtu.be/Upb30KUOVCM

Code analysis

Code analysis can be found in this comment.