`Authorization::value` and `WwwAuthenticate::value` can violate ASCII invariants

`Authorization::value` uses `HeaderValue::value` with the claim
that the internal string is ASCII, but `Authorization::new` and
`Authorization::set_credentials` accept arbitrary `String` credentials without
validation. As a result, safe code can construct a header value containing
non-ASCII UTF-8 while the implementation assumes ASCII.

`WwwAuthenticate::new` and `WwwAuthenticate::set_realm` similarly accepts arbitrary `String` input, so `WwwAuthenticate::value` can also produce a header value that violates the crate’s documented ASCII invariants.

This issue has not been confirmed as Undefined Behavior, but the unsafe
justification in `Authorization::value` and `WwwAuthenticate::value` appears incorrect and can produce values outside the expected ASCII-only constraints.

The http-types crate is unmaintained and the issue is unlikely to be fixed.