Panic on some websites with async-h1 backend: "String slice should be valid ASCII"

On some websites, e.g. http://futureuae.com, surf panics fails with the following error:

> thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: String slice should be valid ASCII

Firefox and curl work fine.

463 websites out of the top million from [Feb 3 Tranco list](https://tranco-list.eu/list/3G6L) are affected.

Tested using [this code](https://github.com/Shnatsel/rust-http-clients-smoke-test/blob/f206362f2e81521bbefb84007cdd25242f6db590/surf-smoke-test/src/main.rs). Test tool output from all affected websites: [surf-should-be-valid-ascii.tar.gz](https://github.com/http-rs/surf/files/6054734/surf-should-be-valid-ascii.tar.gz)

The root cause might be in some dependency and not in surf itself. But the backtrace is useless due to async, and the error message doesn't even point to the line where the panic happens, so I'm reporting it here.

I've only tested the async-h1 backend; I don't know if the other bakends are affected.

Start with the linked surf-smoke-test/src/main.rs and reproduce the panic against futureuae.com using the async-h1 backend. Inspect the reported String slice should be valid ASCII failure and the supplied affected-sites archive; done means affected sites no longer panic, with coverage for the failure case.

Written by the indexing model from the issue text.