krb5_auth: update with correct UPN if needed
    
    The Active Directory KDC handles request case in-sensitive and it might
    not always to possible to guess the UPN with the correct case. We check
    if the returned principal has a different case then the one used in the
    request and updates the principal if needed. This will help using calls
    from the Kerberos client libraries later on which would otherwise fail
    because the principal is handled case sensitive by those libraries.