OKHttp向LeanCloud提交数据不成功

Android客户端的OKHttp向LeanCloud提交数据不成功，请问哪里有问题？
public boolean putBitmap(String json) {
String url = "https://api.leancloud.cn/3.12.3/classes/image";
MD5Utils utils = new MD5Utils();
long time = System.currentTimeMillis();
String header = utils.getMD5(time + Config.appKey);

MediaType JSON
        = MediaType.parse("application/json; charset=utf-8");
RequestBody body = RequestBody.create(JSON, json);
OkHttpClient okHttpClient = new OkHttpClient();
Request request = new Request.Builder()
        .addHeader("X-LC-Sign", header +","+time)
        .url(url)
        .post(body)
        .build();

try {
    Response response = okHttpClient.newCall(request).execute();
    if (response.isSuccessful()) {
        return true;
    }
} catch (IOException e) {
    e.printStackTrace();
    return false;
}
return false;

}
请问问题出在哪里？
关于REST部分有可以学习的demo吗？