this.res._headers has changed in node master
Just compiled Node from the master branch and tried to use koa. It failed because the property values in `this.res._headers` are now arrays instead of strings.

```
$ node
> http = require('http');
> resp = new http.ServerResponse({});
> resp.setHeader('Content-Type', 'foo');
> result = resp._headers
```

In Node 7.x, `result` is `{ 'content-type': 'foo' }`

In Node master (pre-8.0), `result` is `{ 'content-type': [ 'Content-Type', 'foo' ] }`

The change was made here: https://github.com/nodejs/node/commit/c00e4adbb4c6dd5bfbf22a3efcecb63c56958339#diff-286202fdbdd74ede6f5f5334b6176b5cR406 (Node PR: https://github.com/nodejs/node/pull/10558)

@mscdex noticed that it broke express and provided a fix https://github.com/jshttp/fresh/pull/20/files Something similar would be needed for koa.

I'll open this issue to make sure this issue is solved for koa in some way.
