Panic in server when sending non-ASCII header value

Found via fuzzing.

Steps to reproduce:

1. run the server example (`cargo run --example=server`)
2. feed it the bad input (`cat crash | nc localhost 8080`)

`crash` is

```
GET https://example.com HTTP/1.1
Host:ȃ
```

and as base64 (to avoid encoding issues)

```
R0VUIGh0dHBzOi8vZXhhbXBsZS5jb20gSFRUUC8xLjEKSG9zdDrIgw0KDQo=
```

Full backtrace is long, I put it in the below gist.

https://gist.github.com/5225225/dd931721233609b6e6ce39d0705b0119

Start with the `server` example using `cargo run --example=server`, then reproduce the panic by sending the provided `crash` input with netcat. Use the linked gist backtrace to trace the failure, and confirm the server handles the non-ASCII header value without panicking.

Written by the indexing model from the issue text.