Support returning CSV from a JSON query

[guillochon]

At the moment flask-restful has a limitation where it cannot return a mimetype that is different from the requesting mimetype (it ignores the Accept header). A work-around might be possible by designing a custom api class.

For more info see flask-restful/flask-restful#726 and https://stackoverflow.com/questions/47654056/flask-attempting-to-serialize-as-json-despite-mimetype-text-plain.