Fixes RDF export of subobjects

Partial integration of the patch https://gerrit.wikimedia.org/r/#/c/70047/4
from Dbeeson
This is inefficient too, it can be done like this as in BrowseBySubject :
                foreach ( $semanticData->getProperties() as $property ) {
			if ( $property->getKey() === DIProperty::TYPE_SUBOBJECT || $property->getKey() === DIProperty::TYPE_ASKQUERY ) {
                                $this->addSubSemanticData( $store, $property, $semanticData );
                        }
                }

But we got to have a simple method to do this