Change the default value of "use native types" flag to false

This addresses RDF-ISSUE-129. See http://www.w3.org/2011/rdf-wg/track/issues/129

@msporny @niklasl @gkellogg @dlongley @sandhawk What should we do with the tests now? Should we duplicate (the ones affected by this change) and specify the value of the flag in the test?