Slime NBT tags being ignored when spawning normal slimes

When spawning a slime in using the command:

/summon Slime ~ ~1 ~ {Size:1,Attributes:[{Name:generic.maxHealth,Base:10},{Name:generic.movementSpeed,Base:3},{Name:generic.attackDamage,Base:20},{Name:generic.knockbackResistance,Base:2}]}

The slime spawns as a normal slime with the knock-back resistance only applied. The slime still dies in four hits of a player's empty hand and also doesn't hit any harder than normal slimes of that size, as well as not seeming to move any faster than normal, though that part I can kind of understand.