Fix '#' encoding issue in SMWExporter::findDataItemForExpElement

Fixes the issue outlined in https://github.com/SemanticMediaWiki/SemanticMediaWiki/pull/378#issuecomment-48002631
'-23' === '#' but the encoded '-23' is never returned from Sparql query that's why the value display wouldn't work for subobjects because 'Foo#Bar' is returned and not 'Foo-23Bar'.

This has been here for ages and wasn't tested until now since it only matters for when dealing with subobjects.