Support XML in- and ouput (and content negotiation)

[kleijnweb]

Currently only supports JSON.

Content Negotiation

Clients not sending an Accept header will receive content in the format first listed under produces for that operation, or if not available, application/json.

If the Accept header excludes application/json and produces is not set or does not include application/json, an empty response with status code 406 is returned.

If a client sends content other than application/json and the content-type is not listed in the operation's consumes listing, an empty response with status code 406 is returned.