Mobs such as the skeletons and zombies drop armour every time, and it's not just a piece it's the whole suit. I have a spawner I that I use in survival and I have about 2 small chests full of armour, I threw the rest in a lava pool. I have also tried it in creative and it works there too.

To reproduce in creative:
1. Get zombie or skeleton spawn egg
2. Spawn in a zombie or skeleton with armour
3. Kill it
4. All armour is there