cohttp(client): Host header field should be the first header field

From https://www.rfc-editor.org/rfc/rfc9110#section-7.2

>A user agent that sends Host SHOULD send it as the first field in the header section of a request. For example, a GET >request to the origin server for <http://www.example.org/pub/WWW/> would begin with:
>
>```
>GET /pub/WWW/ HTTP/1.1
>Host: www.example.org
>```
