Always mangle incoming Content-Type, even if payload is empty

This lets client consistently use a `application/octet-stream` content type to applications behind `fernet-rack`, without having to adjust the content type for whether or not there is an actual payload (which must happen somewhere, since, e.g., [pliny](https://github.com/interagent/pliny) wigs out if there's a non-JSON content-type even if the body is empty).