accept query parameter as alternative to header

[callum-oakley]

Supports specification of media type via an accept query parameter, which takes the same values as the accept header, but can be used in contexts where the user has no control over headers.

In the original issue there was discussion of using a format parameter and mapping back to a media type from there, but I'm not sure there's any advantage to introducing this extra logic over reusing the existing logic for the accept header? Open to be convinced.

e.g.

$ curl "localhost:3001/v1/sparql/live?accept=application/json&query=$(urlq 'select (count(*) as ?c) where { ?s ?p ?o }')"
{
  "head" : {
    "vars" : [
      "c"
    ]
  },
  "results" : {
    "bindings" : [
      {
        "c" : {
          "datatype" : "http://www.w3.org/2001/XMLSchema#integer",
          "type" : "literal",
          "value" : "28252"
        }
      }
    ]
  }
}

closes #625

[callum-oakley]

Good point, updated the swagger doc (and added another test for the get draftset data case).

[RicSwirrl]

I'm happy to just pass the header as a parameter called accept. Does the job!