Add TryFrom<HttpRequest<()>> for Request

[Gerhut]

http::Request<()> is a common pattern of requests without body in the http library, but it cannot be converted into reqwest::Request. This PR adds such case.

[rlidwka]

in #2307, I did another implementation, which is much simpler, but stores body differently (None vs Some(Bytes::empty())), please check it out and give comment if any

I did my own investigation into this issue before I found this PR, so I'd be happy if either one is merged