Implemented 'application/sparql-query' support for SPARQL 1.1 endpoints.

This is a feature introduced in the SPARQL 1.1 Protocol specification
whereby the SPARQL query string may be submitted directly as the body
of the HTTP request, avoiding the complication and time/space overhead
of URL encoding/decoding the query string as was required by the
previous version of the protocol.

For maximum compatibility, this client implementation still defaults
to using the SPARQL 1.0 protocol; to enable the new 1.1 protocol
support, configure your SPARQL::Client instance as follows:

    SPARQL::Client.new(endpoint, :protocol => 1.1)

See: http://www.w3.org/TR/sparql11-protocol/#query-via-post-direct