cas: Make it easier to override finalStep() in subclasses.

This patch makes it easier to override the finalStep()-function in
subclasses by making sure that the casValidation()-function can be used
from subclasses.

Patch by Josselin Jacquard.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2886 44740490-163a-0410-bde0-09ae8108e29a