public IEnumerator PostJson <T>(string url, IRestJsonResponseHandler <T> handler, object payload)
        {
            using (UnityWebRequest req = new UnityWebRequest(url))
            {
                req.method = UnityWebRequest.kHttpVerbPOST;
                req.SetRequestHeader("Content-Type", "application/json");
                var uploader = new UploadHandlerRaw(Encoding.UTF8.GetBytes(ToJson(payload)));
                uploader.contentType = "application/json";
                var downloader = new DownloadHandlerBuffer();
                req.uploadHandler   = uploader;
                req.downloadHandler = downloader;
                yield return(req.SendWebRequest());

                if (req.isHttpError)
                {
                    handler.OnHttpError(req.responseCode, req.error);
                }
                else if (req.isNetworkError)
                {
                    handler.OnFailure(req.error);
                }
                else
                {
                    handler.OnSuccess(FromJson <T>(req.downloadHandler.text));
                }
            }
        }
        public IEnumerator GetJson <T>(string url, IRestJsonResponseHandler <T> handler)
        {
            using (UnityWebRequest req = UnityWebRequest.Get(url))
            {
                yield return(req.SendWebRequest());

                if (req.isHttpError)
                {
                    handler.OnHttpError(req.responseCode, req.error);
                }
                else if (req.isNetworkError)
                {
                    handler.OnFailure(req.error);
                }
                else
                {
                    handler.OnSuccess(FromJson <T>(req.downloadHandler.text));
                }
            }
        }