Fix to ensure that default namespace can round trip through JSON-LD

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1596878 13f79535-47bb-0310-9956-ffa450edef68
Expand tests for JSON-LD to include namespace round tripping verification

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1596879 13f79535-47bb-0310-9956-ffa450edef68
Improve how HttpQuery handles HttpException from HttpOp machinery

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1596880 13f79535-47bb-0310-9956-ffa450edef68