Added support for "ntriples" for when typo is forgotten
Added support for "ntriples" for when typo is forgotten

Even though “ntriples" should not be used, it’s a common typo. I deem
it common enough to support it. Also, rather than returning null, we
now return “.unknown”. Even though that should never happen, it is at
least better than “null”…

CNGL-repo/r2rml