HTTP2 server does not respond
```js
var http2 = require('http2');
var Koa = require('koa');

var app = new Koa();

app.use(async (ctx) => {
    ctx.body = 'Hello World';
});

http2.createServer(app.callback()).listen(3000);
```
try to hit the server in browser `http://localhost:3000` and get error:
```bash
The localhost site sent an invalid response. ERR_INVALID_HTTP_RESPONSE
```

node: v14.13.0
koa: 2.13.0