Equate queryset model instances with deffered fields to their parent model class when resolving UI model columns.

This will allow models using .defer or .only optimizations to render properly on list views.

mayan/apps/navigation/classes.py

@@ -349,6 +349,9 @@ class SourceColumn(object):
                 try:
                     return cls._registry[source.__class__]
                 except KeyError:
+                    try:
+                        return cls._registry[source._meta.parents.items()[0][0]]
+                    except IndexError:
                         return ()
         except TypeError:
             # unhashable type: list