Mob tag Invulnerable does not work

Put the summary of the bug you're having here

What I expected to happen was...:
What I expected to happen was the mob should not take damage of the player.

What actually happened was...:
What actually happened was the mob has take damage of the player.

Steps to Reproduce:
1. Create new world of type superflat.
2. Put command "give @a command_block"
3. Put command block in the floor and imput that command: "summon Pig ~ ~1 ~

{Invulnerable:1}

"
4. Energize the command block with redstone block
5. Hit the mob with any weapon and see that dies