Add missing test coverage for XMLResponseSerializer
Add missing test coverage for XMLResponseSerializer

Currently we have no direct coverage of this, so add some tests
prior to refactoring in a subsequent patch.

Change-Id: I20242e60b84f9265cdc7afef2deabee8235e287c
Related-Bug: #1291079

openstack/heat,dragorosson/heat,cryptickp/heat,jasondunsmore/heat,pratikmallya/heat,rdo-management/heat,rdo-management/heat,dragorosson/heat,miguelgrinberg/heat,gonzolino/heat,takeshineshiro/heat,srznew/heat,pshchelo/heat,maestro-hybrid-cloud/heat,maestro-hybrid-cloud/heat,cwolferh/heat-scratch,steveb/heat,openstack/heat,gonzolino/heat,cwolferh/heat-scratch,miguelgrinberg/heat,noironetworks/heat,cryptickp/heat,rh-s/heat,redhat-openstack/heat,steveb/heat,takeshineshiro/heat,redhat-openstack/heat,pratikmallya/heat,dims/heat,dims/heat,srznew/heat,jasondunsmore/heat,rh-s/heat,noironetworks/heat,pshchelo/heat