Melee damage with zombies

If a zombie/zombie pigman picks up a sword, or has one, the damage they deal is their normal melee damage + the sword damage, so if their melee damage is 2, and the sword damage is 2.5, the damage they deal will be 4.5, instead of 2.5 that it should be.

Steps to reproduce:

1. Create a survival world
2. Find a zombie with a sword/give a sword to a zombie
3. The damage will be the difficulty damage + the sword damage