Broken CORS w/ WebSocket example

### What happened?

When connecting via Apollo WebSocket link, it fails to connect due following error:
`Error during WebSocket handshake: Unexpected response code: 403`

Checking server logs, following error is spotted:
```
[cors] 2020/09/14 12:44:17 Actual response added headers: map[Access-Control-Allow-Credentials:[true] Access-Control-Allow-Origin:[http://localhost:8080] Vary:[Origin]]
2020/09/14 12:44:17 unable to upgrade *http.response to websocket websocket: request origin not allowed by Upgrader.CheckOrigin:
2020/09/14 12:44:17 http: superfluous response.WriteHeader call from github.com/99designs/gqlgen/graphql/handler/transport.SendError (error.go:15)
```

### What did you expect?

That it works...

### Minimal graphql.schema and models to reproduce

https://gqlgen.com/recipes/cors/

### versions
- `latest gqlgen version`?
- `1.15`?
- go modules