$Subj was omitted from https://github.com/gophercloud/gophercloud/pull/2781 because it had not been implemented in Ironic at that point. It goes re-introduced in https://review.opendev.org/c/openstack/ironic/+/892661, so now we can cover this gap. Follow-up to https://github.com/gophercloud/gophercloud/issues/2612