JENA-663 : Use general Accept header

1/ Accept header to prefer Turtle, then NT, JSON-LD and RDF/XML (then anything).
2/ Allow "Accept:" and outgoing "Content-type:" to be set by the application.


git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1582408 13f79535-47bb-0310-9956-ffa450edef68