spec on sparql/query no longer allows VALUES binding

[RickMoynihan]

e.g. you used to be able to target a sparql query like this:

SELECT * WHERE { 
  VALUES (?s ?p) { (:s-1 rdfs:label)
                               (:s-2 skos:notation) }
  ?s ?p ?o .
}

These queries now raise a spec error.

We should broaden the spec to cover this case.