[fuseki] Enable application/rdf+json as rdf representation

[Niclas Hoyer]

Hi,

I created a small pull request [1] related to Fuseki. I enabled 
"application/rdf+json" as representation for RDF data.  The WebContent 
class already had the contentTypeRDFJSON property, so only small changes 
in DEF.java were necessary. Are pull requests accepted there? Or should 
I submit a patch elsewhere?

[1] https://github.com/apache/jena/pull/1/files

Regards,
Niclas