Fix serializing an xml producer by itself.
Fix serializing an xml producer by itself.

Applications should use the xml producer's _original_type and
_original_resource properties and coerce it to a playlist or tractor to
serialize the entire graph of nodes.

xzhavilla/mlt,siddharudh/mlt,xzhavilla/mlt,xzhavilla/mlt,j-b-m/mlt,wideioltd/mlt,wideioltd/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,siddharudh/mlt,zzhhui/mlt,xzhavilla/mlt,wideioltd/mlt,mltframework/mlt,xzhavilla/mlt,wideioltd/mlt,zzhhui/mlt,zzhhui/mlt,anba8005/mlt,wideioltd/mlt,siddharudh/mlt,j-b-m/mlt,anba8005/mlt,siddharudh/mlt,zzhhui/mlt,wideioltd/mlt,anba8005/mlt,wideioltd/mlt,xzhavilla/mlt,j-b-m/mlt,mltframework/mlt,zzhhui/mlt,zzhhui/mlt,j-b-m/mlt,siddharudh/mlt,wideioltd/mlt,j-b-m/mlt,zzhhui/mlt,anba8005/mlt,anba8005/mlt,siddharudh/mlt,siddharudh/mlt,j-b-m/mlt,siddharudh/mlt,xzhavilla/mlt,wideioltd/mlt,siddharudh/mlt,mltframework/mlt,anba8005/mlt,anba8005/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,zzhhui/mlt,xzhavilla/mlt,j-b-m/mlt,xzhavilla/mlt,anba8005/mlt,anba8005/mlt,j-b-m/mlt,zzhhui/mlt,mltframework/mlt