Fix for subobjects for RDF stores

```php
	$this->doSparqlFlatDataUpdate( $semanticData );

	foreach( $semanticData->getSubSemanticData() as $subSemanticData ) {
		 $this->doSparqlFlatDataUpdate( $subSemanticData );
	}
```
This is sufficient because the "main" subject has a reference to `<swivt:specialProperty_SOBJ rdf:resource="http://.../Special:URIResolver/Lorem_ipsum-23_2e426edb2eec26f5007e895b76ee2a33"/> ` linking it to a subobject while the subobject has a reference to `<swivt:masterPage rdf:resource="http://.../Special:URIResolver/Lorem_ipsum"/>`.

When `doSparqlDataDelete` is executed those with `SMWExporter::getSpecialNsResource( 'swivt', 'masterPage' )` are feed to the `deleteContentByValue` and deleted as well.

PS I did a manual check.
PSS: Probably want to write a test.