Fixes RDF export of subobjects

Partial integration of the patch https://gerrit.wikimedia.org/r/#/c/70047/4
from Dbeeson
This should go into the Store where the original `getSemanticData ` is left as is and a new method like `getAllSemanticData` or `getCompleteSemanticData` is introduced that contains the additional part.

Tests can be borrowed from BrowseBySubject round trip for the addSubSemanticData part.