Summary: Zombies will always pursue the Player first, even when nearby Villagers are closer to the Zombie than the Player is. It has also been observed that Zombies who are pursuing Villagers, will randomly stop attacking the Villager and will began attack the Player. Both these issues has only been tested on Hard difficulty. Can probably attribute it to improvements on pathfinding since 15w49a.

How to Reproduce:
1. Place a Monster Spawner and set the spawning mob to a Zombie.
2. Get far away enough so that you aren't triggering the spawner, but not too far that you can't get a Villager near the spawner.
3. Spawn a Villager, make sure it is in the trigger radius of the spawner.
4. Hit the Villager a few times until there is enough space for you to get inside the trigger radius, also be sure the Villager is between you and the spawner, but make sure the spawner is visible.
5. Note how the Zombie pursues you instead of the Villager.
6. If you run far away enough, the Zombie might pursue the Villager instead, but if you get into the detection range, the Zombie will stop attacking the Villager and will begin to pursue the Player.