In the latest snapshot, the new general mob AI makes some mobs run from creepers when they are about to explode. However, they seem to not do it very often. I've spawned a creeper, with several zombies, skeletons, creepers, and witches around it a number of times, ignited it, and about an average of 1-3 out of ten actually ran. It may have been improved since 14w11, may be intended.