Fail on map_username for AD

[davelebbing]

v5.0 build 7096

When setting map_username to sn for an AD authentication environment it was not possible to sign in with the user's last name as expected. This was for a user who was logging in for the first time on this setup. Logging in the default way with the username did succeed and create a new user account. The value for sn was confirmed to be shown as expected in the XCreds log after signing in.