Literals with attributes should be added as rdf:value nodes

As per http://www.w3.org/TR/rdf-primer/#rdfvalue
```
<rdf:RDF
  xmlns="http://purl.org/dc/elements/1.1/"
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:bib="http://purl.org/net/biblio#">
  <bib:Document rdf:about="something">
      <language xsi:type="dcterms:ISO639-3">eng</language>
  </bib:Document>
</rdf:RDF>
```
should result in something like
```
<something> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://purl.org/net/biblio#Document> .
<something> <http://purl.org/dc/elements/1.1/language> _:n2 .
_:n2 <http://www.w3.org/2001/XMLSchema-instancetype> "dcterms:ISO639-3" .
_:n2 <http://www.w3.org/1999/02/22-rdf-syntax-ns#value> "eng"
```

Pre-patch, this results in
```
<something> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://purl.org/net/biblio#Document> .
<something> <http://purl.org/dc/elements/1.1/language> _:n0 .
_:n0 <http://www.w3.org/2001/XMLSchema-instancetype> "dcterms:ISO639-3"
```