Updating a world created before 1.9 does not make iron golems knockback resistance

Summary:
In 1.9 it was made that iron golems have knockback resistance. Upgrading a world created before 1.9 does not make golems knockback resistance.

Steps to reproduce:
Use this world to go straight to step 3

.

1. Create a world in version before 1.9.

2. Create an iron golem.

3. Upgrade the world to the latest version.

4. Hit the iron golem.

Observed results:
Iron golems do not have knockback resistance.

Expected results:
Iron golems have knockback resistance.