Set default mimeType for NtriplesRDFDatastream to 'application/n-triples'
per W3C recommendation.
http://www.w3.org/TR/n-triples/#n-triples-mediatype
Fixes #464
Merge pull request #465 from dchandekstark/n-triples-mime-type

Set default mimeType for NtriplesRDFDatastream to 'application/n-triples'