`getAcceptInfo` can raise an unlisted exception: ref ValueError
If I try to compile `httpserver.nim` like that:
```bash
nim c chronos/apps/http/httpserver.nim
```
I get the following error:
```bash
nim-chronos/chronos/apps/http/httpserver.nim:563:16 Error: getAcceptInfo(getString(request.headers, AcceptHeaderName, "")) can raise an unlisted exception: ref ValueError
```

I trigger the problem using `nim v1.6.14` ( same result with `nim v1.6.18` )