__json__ method of LDAPObject may inadvertently append a list of possible
objectclasses to a list of basic objectclasses and thus change a behavior
of all subsequent LDAPSearch command. The command may only return objects
where all "possible" objectclasses are present and thus returning an
incomplete list.
Make sure that the LDAPObject object_class list is not modified during
the __json__ method.
https://fedorahosted.org/freeipa/ticket/2906