typeof this.res.hasHeader === 'function' is meaningless
![image](https://user-images.githubusercontent.com/8686739/91116773-b51dea00-e6bf-11ea-8b7d-75b63fa013c4.png)
in the nodejs http and http2 module, hasHeader is **function**, In the Koa, it must be also a **function**.
by the way, why not support very old node version < 7.7, that is not LTS node version. i think it is not useful