saveAsNTriplesFile problem (+fix proposal) #237

Hi, I think I've found a problem with saveAsNTriplesFile : URIs with a very common namespace (like http:// www. w3.org/ 1999/ 02/ 22-rdf-syntax-ns#) are abbreviated, so the written N-Triples files may hold triples such as:
<http://www.my.namespace.org/subject> rdf:type <http://www.my.namespace.org/object> .

This is because in JenaTripleToNTripleString.scala we have :

class JenaTripleToNTripleString
    extends (Triple => String)
    with java.io.Serializable {
  override def apply(t: Triple): String = s"${FmtUtils.stringForTriple(t)} ."
}

so stringForTriple(t) will use commonly known namespaces.

I suggest the following fix, which simply consists in using an empty PrefixMappingImpl object :

private class JenaTripleToNTripleString
    extends (Triple => String)
      with java.io.Serializable {
    override def apply(t: Triple): String = s"${FmtUtils.stringForTriple(t,new PrefixMappingImpl())} ."
  }

Cheers