[SPARQLConnector] CONSTRUCT queries should use RDF media types in Accept header

[smeckler]

Hi,
I would like to change the HTTP Accept header that the SPARQLConnector sends for SPARQL CONSTRUCT queries.

current behavior: SPARQLConnector sets the Accept header to the user-defined 'returnFormat' or default media types. The resulting Accept header uses media types for SPARQL results, e.g. application/sparql-results+json for returnFormat=json

desired behavior: SPARQLConnector selects only RDF graph media types, e.g. application/ld+json for CONSTRUCT or DESCRIBE queries which return a graph

I created a small PR for checking the query type and selecting the media types accordingly.

SPARQL 1.1 protocol

The response body of a successful query operation with a 2XX response is either:
    a SPARQL Results Document in [XML](https://www.w3.org/TR/rdf-sparql-XMLres/), [JSON](https://www.w3.org/TR/sparql11-results-json/), or [CSV/TSV](https://www.w3.org/TR/sparql11-results-csv-tsv/) format (for SPARQL Query forms [SELECT](https://www.w3.org/2001/sw/DataAccess/rq23/#select) and [ASK](https://www.w3.org/2001/sw/DataAccess/rq23/#ask)); or,
    an RDF graph [[RDF-CONCEPTS](https://www.w3.org/TR/sparql11-protocol/#rdf-concepts)] serialized, for example, in the [RDF/XML syntax](https://www.w3.org/TR/rdf-syntax-grammar/) [[RDF-XML](https://www.w3.org/TR/sparql11-protocol/#rdf-xml)], or an equivalent RDF graph serialization, for SPARQL Query forms [DESCRIBE](https://www.w3.org/2001/sw/DataAccess/rq23/#describe) and [CONSTRUCT](https://www.w3.org/2001/sw/DataAccess/rq23/#construct)).

In section 3.1.3, there is an example of an HTTP request with a construct query:

• the request uses the HTTP header Accept: text/turtle, application/rdf+xml
• the response has the HTTP header Content-Type: text/turtle

Thank you for your help!

[WhiteGobo]

As far as i understand, you need per default turtle in Accept. But rdflib currently always uses xml there. With your current PR you can only switch to turtle by defining store.returnFormatright?
I think it would be an idea to extend the ehaviour of sparqlstore to also accept arrays for returnFormatConstruct(See my review, for the name), so that your accept header would contain both xml and turtle

[WhiteGobo]

Also apart of this issue, we need to check that sparqlconnector communicates in its header by default that, xml is accepted. Otherwise, we could change that behaviour by chance, without knowing. i will write an pr with this check and link it here.

[WhiteGobo]

Also i dont understand, why by default, we dont accept every format, that rdflib can comprehend.We can changed that behaviour, by changing sparqlstore.returnFormat to None by default.
(sitenote FORMAT_MIMETYPE_MAP and RESPONSE_TABLE_FORMAT_MIMETYPE_MAP are not a list of all comprehended dialects but a static map, that hopefully reflect, whatever plugins we have abailable.)