【备忘】koa set cookie带中文的解决方案

```
this.cookies.set('test', new Buffer('我是koajs').toString('base64'))
```

参考链接：
[what-character-encoding-should-i-use-for-a-http-header](https://stackoverflow.com/questions/4400678/what-character-encoding-should-i-use-for-a-http-header)