Non ASCII header values causes panic

I came here via an unexpected panic in my [tide webserver](https://github.com/http-rs/tide/), but I think the problem belongs here.
When I'm using a cookie that might be a bit "odd" like `password=Â` , the calling library (async-h1 or tide) is calling `append_header` from request.rs, then `append` and `insert` within the headers/headers.rs file to set that header. On line 54 it says:
`let values: HeaderValues = values.to_header_values().unwrap().collect();`
which fails and panics with a `Result::unwrap() on an Err value: String slice should be valid ASCII`.
I think those functions should have a result of type `Result` if these functions can fail in such circumstances.