"double callback bug" is logged when http2 error occur

If I request an non-http2 server with http2 superagent throws error with `superagent: double callback bug`.
Wondering if this is an expected behavior for http2?

#### Sample code
```javascript
superagent
.get('https://github.com')
.http2()
.then(console.log)
.catch(console.log)
```