Zombies don't try to break doors to get villagers or players. No breaking animation on the door, no knock sound, they are simple not trying to break the doors. obs: /gamerule mobGriefing has not set to false. The same bug persists from 1.7x to the 1.8.7 versions. They don't break the doors even in hard difficulty. Besides the animation of the door breaking is missing in all difficulties, because they are not trying to break the doors.

EDIT:
FALSE BUG REPORT
The zombies only have changed they mechanics.. see more about the new break door mechanics here: http://www.reddit.com/r/Minecraft/comments/2qqgcx/til_minecraft_18_changed_how_zombies_break_doors/

You Can summon a Zombie with can break doors with the command /summon Zombie ~ ~ ~

They naturally spawn more rare now, making villages even in the hard difficulty more safe from zombies.