Set correct Content-type header in SPARQL endpoint (allows using from external tools)

This release contains a smaller - but potentially important - fix to the SPARQL endpoint included in RDFIO, so that it provides the correct Content-type header for SPARQL result documents by default, allowing to query the SPARQL endpoint in RDFIO with external tools.

See the commit for details: 060c63d

The functionality verified by querying a local Semantic MediaWiki / RDFIO installation from SWI-Prolog, using its bundled SPARQL client. See screenshot: