CAS-1331: 'Adapt' pac4j module to new CAS 4 authentication API

I've made complete tests based on the new CAS 4 authentication API (4.0.0-RC2) and there are some problems with the pac4j module :
- a ```NullPointerException``` has been fixed
- the "aborted" authentication is now properly handled in the ```ClientAction``` class
- the ```ClientAuthenticationMetaDataPopulator``` has been simplified as it doesn't need any more to populate user's attributes (already populated by the authentication handler).