ios 返回值非JSON处理

当返回值不是JSON格式时，需要添加一行如下代码：

  manager.responseSerializer = [AFHTTPResponseSerializer serializer];

否则会报如下错误：

    Error Domain=NSCocoaErrorDomain Code=3840 "JSON text did not start with array or object and option to allow fragments not set." UserInfo={NSDebugDescription=JSON text did not start with array or object and option to allow fragments not set.}