nope you did it wrong, you need to set your mob render to biped, and model if you have to biped
here is my tutorial, for custom entity with working item holding and own biped model
http://rolikstuff.forumbuild.com/viewtopic.php?f=10&t=39:
and you only need to add this this.setCurrentItemOrArmor(0, new ItemStack(Items.golden_sword));
but not held item, it will work if you have biped or zombie render