JENA-598 - Use URL file extension if no content type (or it's text/plain) by using RDFDataMgr.determineLang.

And various null point situations handled more gracefully (found during debugging).


git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1546450 13f79535-47bb-0310-9956-ffa450edef68