Support for SPARQL media type

[jaw111]

To help with debugging queries, it would be handy to be able to retrieve the actual instantiated query text that grlc sends to the endpoint based on the parameterization.

Whilst that is probably something that could be logged, it would potentially make the logfile rather verbose.

Instead it would be nice if the query text could be retrieved by sending header Accept: application/sparql-query in the request.