Don't assume that plugin attributes and objectclasses are lowercase
    
    A user wrote their own plugin to add custom attributes which was
    failing with an incorrect error that the attribute wasn't allowed.
    
    It wasn't allowed because it wasn't being treated as case-insensitive
    so wasn't being found in the schema.
    
    https://pagure.io/freeipa/issue/8415
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>