Fixes a panic that would ocurr as we proxy 4xx responses. When this happens and the content type of the response is JSON we try to check if the response has a "message" key. Then, we assume that the key will contain a value of string but we don't take into account that this value can potentially be `null`.
This adds a type assertion check to to this assumption so that we can keep the original JSON body as the response if we're unable to extract an `message`.
5b64c4f684