Http2ServerResponse.statusMessage is not writeable
Node 8.4.0 has introduced the "http2" module as part of node core, which you can use if you launch node with `--expose-http2`.

When using it with koa, however, I get an error:

```
TypeError: Cannot set property statusMessage of #<Http2ServerResponse> which has only a getter
```

The error comes from /lib/response.js, line 88. HTTP/2 does not have status messages, so Node throws an error when you try to set one.

This is the code I'm using:

```javascript
const http2 = require('http2');
const fs = require('fs');
const Koa = require('koa');
const Router = require('koa-router');

const app = new Koa();

const options = {
  key: fs.readFileSync('server.key'),
  cert: fs.readFileSync('server.crt')
};

http2.createSecureServer(options, app.callback()).listen(3000);

const router = new Router();
router.get('/', (ctx) => ctx.body = "response");

app.use(router.routes());
```
