cohttp 5.3.0: `Cookie.Set_cookie_hdr.extract` only work with lower case `set-cookie` and not uppercase `Set-Cookie`

My work around is:
```ocaml
    let header =
      Header.add_multi header "set-cookie" @@ Header.get_multi header "Set-Cookie"
    in
```

I noticed that this has already been fixed in master with `Http.Header.Private.caseless_equal` but it'll be nice if the fix can be backported.

Thank you :)