Default route does not work.

I created a hostrouter like in the readme.

```go
r := chi.NewRouter()
r.Get("/test", func(writer http.ResponseWriter, request *http.Request) {
//...
})

hr := hostrouter.New()
hr.Map("", r) // default
```

---

But always got 404s. Changed it to wildcard (`hr.Map("*", r) `) and it worked. This took my quite some time to figure out, don't know if I'm missing something or not. But after inspecting the hostrouter .ServerHttp function, I suspect that is no "default" at all?

Maybe `hr.Map("", apiRouter()) // default` should be removed from the readme, or if I'm missing something then maybe update the readme to make it clearer?

Start with the default-host example in the README and inspect the hostrouter ServerHttp function to compare empty-host and wildcard mappings. Reproduce the 404 using the provided chi router, then confirm whether the behavior or only the documentation needs changing; done means the default mapping works as documented or the README clearly explains the required wildcard.

Written by the indexing model from the issue text.