服务器返回的数据是application/xml格式的，AFN默认处理的是JSON格式的所以会报错
解决方法：添加@“application/xml”
AFHTTPSessionManager *session = [AFHTTPSessionManager manager];
session.responseSerializer.acceptableContentTypes = [NSSet setWithObjects:@"application/json", @"text/json", @"text/javascript",@"text/html", @"application/xml", nil];
