CAS-1352: Usename attribute should not be required in the list of allowed attributes

Specifying the username attribute in the service registry for a given service currently requires that the same attribute exist in the list of allowed attributes.

https://issues.jasig.org/browse/CAS-1352
Updated test cases to reflect unallowed attributes for services can be selected as the username attribute.
Merge pull request #314 from Unicon/CAS-1352

CAS-1352: Usename attribute should not be required in the list of allowed attributes