Rename command doesn't work if literals are xsd:strings

If the input has xsd:strings on literals (as is common for many
ontologies like GO):

```turtle
<http://purl.obolibrary.org/obo/NCBITaxon_2>
<http://www.w3.org/2000/01/rdf-schema#label> "Bacteria"^^xsd:string .
```

then rename fails: it adds a new name but does not delete the old

This seems to be a more general issue