application/ld+json emits prefix as NamedNode

normally prefixes-parsed emits the prefix like this 
```
onPrefixesParsed {rdfs: 'http://www.w3.org/2000/01/rdf-schema#'}
````

except application/ld+json emits a NamedNode instead of a string. 

```
rdf-editor.component.ts:39 format application/ld+json
rdf-editor.component.ts:57 onPrefixesParsed {rdfs: NamedNode}
```