Update classService.php

You never pass through User Credentials so if you want to do something like get classes with the class availability counts which require a user you'll never get them. Although this will break if you don't pass through a user at all.

I think this should be rewritten to return null if you don't pass through User Credentials specifically or you don't setup any user credentials using SetDefaultCredentials.

throw new Exception('No source user credentials supplied, and no default user credentials stored');