[server] content negotiation
implement looking at the content type and accept-encoding headers. see exception in bootstrap.php i suggest that we require PUT / POST to use a content type listed in Accept. otherwise we could need two different serializers, one for request and one for response. use stati 406 (not acceptable) for that. and 415 when content-type of request is not supported.
implement a second serializer so that we can see the thing in action.