Download file Not Callback with block

[vbonluk]

提出问题前请先确认完成了下列几项步骤 New Issue Checklist

• [ ☑️] 我已经阅读过 贡献指南 I have read and understood the CONTRIBUTING guide
• [ ☑️] 我已经阅读过 程序文档 I have read the Documentation
• [☑️ ] 我已经证实这个问题来源于 YTKNetwork 本身，而不是其所依赖的 AFNetworking I have confirmed that this issue is caused by YTKNetwork, not AFNetworking
• [ ☑️] 我已经在项目的 问题列表 中搜索过并且没有找到类似问题 I have searched for a similar issue in the project and found none

问题描述 Issue Description

pic 👆 loading so slow ，please wait

I using your repo to download some file , but found some error.

Like this:

never callback with block.

But callback in delegate

Please fill in the detailed description of the issue (full output of any stack trace, compiler error, ...) and the steps to reproduce the issue.