I'm not completely keen on hardcoding this as a default. If anything, I guess it could be added to the options in the `if (allOptions.type == 'json') {` branch of post/put, even though I'm getting a bit confused with `type` (is it meant for the data that is sent, or the data we "Accept" as a response).

In practice, we might want to send either JSON or the argo media type as Content-Type, depending on the context. But might be safe to use application/json for now in post/put when sending JSON.