Fix whitelist to be modifiable from behaviors to work with validate.

Allows behaviors to modify/extend the whitelist when adding new fields.
Otherwise those will be disregarded for validation - which is quite a deficiency in behavior functionality IMO.

Initially I thought of this as a bugfix. 
But it seems the behaviors have not been able to modify the whitelist so far in 2.x - so this might be more a change of behavior for some.

Actually, I wanted to keep the ValidationRuleBehavior test class separate in the test_app folder, but it wasn't loaded.
Anything missing there?