Iron Golem custom attribute attack.damage ignored by server.

I made a custom Iron golem and tryed to change his attack damage, the attack damage did not affect the iron golem and in the server log it says

[07:06:26] [Server thread/WARN]: Ignoring unknown attribute 'generic.attackDamage'

and here is the command i used to spawn a iron golem

/summon Sheep ~-1 ~-1 ~ {CustomName:"jeb_",CustomNameVisible:1,Color:15,Attributes:[{Name:generic.maxHealth,Base:900},{Name:generic.movementSpeed,Base:0.1},{Name:generic.attackDamage,Base:10},{Name:generic.followRange,Base:50},{Name:generic.knockbackResistance,Base:10}],Invulnerable:1,HealF:900,ActiveEffects:[{Id:6,Amplifier:5,Duration:19999980},{Id:10,Amplifier:5,Duration:19999980},{Id:21,Amplifier:5,Duration:19999980}],Riding:{id:"VillagerGolem",CustomName:"jeb_ S.W.A.T",CustomNameVisible:1,PlayerCreated:0,Attributes:[{Name:generic.maxHealth,Base:290},{Name:generic.movementSpeed,Base:0.5},{Name:generic.attackDamage,Base:15},{Name:generic.followRange,Base:100},{Name:generic.knockbackResistance,Base:0.5}],HealF:290,ActiveEffects:[{Id:5,Amplifier:10,Duration:19999980},{Id:8,Amplifier:1,Duration:19999980},{Id:10,Amplifier:2,Duration:19999980},{Id:11,Amplifier:2,Duration:19999980}]}}

Steps to reproduce problem=

1. get a private server

2. get a command block

3. put my command in the command block.

4. Summon the iron golem

5. look at your server log

6. It should say

   [Server thread/WARN]: Ignoring unknown attribute 'generic.attackDamage'