404 response for HEAD request with no Accept: 

Steps to reproduce:

> $ curl -I https://crates.io/
> HTTP/1.1 404 Not Found
> <...>
> Content-Type: application/json; charset=utf-8

With Accept: text/html everything is okay:

> $ curl -H "Accept: text/html" -I https://crates.io/
> HTTP/1.1 200 OK
> <...>
> Content-Type: text/html
