py3: add_entry_to_group: attribute name must be string not bytes

With bytes as attribute name pyldap raises type error

https://fedorahosted.org/freeipa/ticket/4985

Reviewed-By: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Jan Cholasta <jcholast@redhat.com>

ipaserver/plugins/ldap2.py

@@ -417,7 +417,7 @@ class ldap2(CrudBackend, LDAPClient):
         # update group entry
         try:
             with self.error_handler():
-                modlist = [(a, self.encode(b), self.encode(c))
+                modlist = [(a, b, self.encode(c))
                            for a, b, c in modlist]
                 self.conn.modify_s(str(group_dn), modlist)
         except errors.DatabaseError: