Support all RDF serializations

[jirihelmich]

For upload, the mission is clear, based on file extension, tell Jena, which type of serialization it is.

What about URL download? I don't like the idea of switching to Virtuoso internal functions now... But without it, we won't have TriG support for URL download.

[jirihelmich]

https://any23.apache.org/developers.html

[jirihelmich]

This is fully Virtuoso Graph Store protocol dependent. Either downloaded RDF or uploaded RDF is just forwarded. In case of download the only additional thing is that I forward the retrieved content type.

I added some detection for the case that the content type is not provided, I can make it always decide by the filename.

Removing from the milestone as this cannot be fixed easily.