Naturally-spawned villagers farm crops regardless of profession

All villager professions farm crops when only the farmer profession should farm.
Steps to reproduce:
1. Creat a world with a village in it (the seed "Evanescence" has a village at spawn)
2. Watch all villagers farm crops (use bonemeal to fully grow crops if needed)

EDIT by Zeb: Note that, as far as I can tell, this only affects villagers that spawned naturally in 1.0.4.0. Villagers that spawned naturally in 1.0.3 or villagers spawned using spawn eggs or the /summon command are NOT affected by this bug.