Wither Skeletons should have sheilds

While wither skeletons have been pretty tough enemies, they aren't really that interesting to fight. They also are easy to cheese, just by blocking any area for their head to go through, and fight them while they stand there. Giving them shields will fix this. Wither skeletons will use their shields intelligently to block the player's attacks. When they're blocked, they'll keep their shield up and play more defensively. This will make combat in nether fortresses more interesting, and less annoying, and spammy.