Fixed wilbur db saving bug

Without this fix I receive following output when saving wilbur db
```rdf
 <NIL> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral> .
 <urn:uuid:806de881-a984-11e2-9f40-1803736abf66> <http://xmlns.com/foaf/0.1/firstName> <NIL> .
 <NIL> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral> .
 <urn:uuid:806de881-a984-11e2-9f40-1803736abf66> <http://xmlns.com/foaf/0.1/age> <NIL> .
 <http://xmlns.com/foaf/0.1/firstName> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/1999/02/22-rdf-syntax-ns#Property> .
 <urn:uuid:5b2bab01-a981-11e2-86bc-1803736abf66> <http://xmlns.com/foaf/0.1/firstName> <NIL> .
 <http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2000/01/rdf-schema#Class> .
 <NIL> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral> .
 <http://xmlns.com/foaf/0.1/age> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/1999/02/22-rdf-syntax-ns#Property> .
 <urn:uuid:5b2bab01-a981-11e2-86bc-1803736abf66> <http://xmlns.com/foaf/0.1/age> <NIL> .
```

and here is correct output after fix 
```rdf
"Test"^^<xsd:string> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral> .
<http://xmlns.com/foaf/0.1/firstName> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/1999/02/22-rdf-syntax-ns#Property> .
<urn:uuid:5b2bab01-a981-11e2-86bc-1803736abf66> <http://xmlns.com/foaf/0.1/firstName> "Test"^^<xsd:string> .
<http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2000/01/rdf-schema#Class> .
"70"^^<xsd:integer> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral> .
<http://xmlns.com/foaf/0.1/age> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/1999/02/22-rdf-syntax-ns#Property> .
<urn:uuid:5b2bab01-a981-11e2-86bc-1803736abf66> <http://xmlns.com/foaf/0.1/age> "70"^^<xsd:integer> .
```