Introduce keep_original_uri to support the proxy without touching the original request URI

[jongkoo25]

Hi, Folks,

Thank you for the awesome project and I really like to use this library.

While using this library, I have a quick suggestion to support more use case, such as proxy. This is similar with #2509.

The project I'm now working on is to send the original request to other downstream if the current server doesn't support it. It is kind of fallback mechanism. In this process, one of the requirement is that we want to keep the original request as it is and let the downstream sees the same requests. The downstream is on the same host. If it is HTTPS request, the main server will terminate it and forward to the downstream service.

In Hyper, the problem is that it enforces making origin_form if we call Hyper Client with is_proxied = true and absolute_form. (Link)

fn absolute_form(uri: &mut Uri) {
    debug_assert!(uri.scheme().is_some(), "absolute_form needs a scheme");
    debug_assert!(
        uri.authority().is_some(),
        "absolute_form needs an authority"
    );
    // If the URI is to HTTPS, and the connector claimed to be a proxy,
    // then it *should* have tunneled, and so we don't want to send
    // absolute-form in that case.
    if uri.scheme() == Some(&Scheme::HTTPS) {
        origin_form(uri);
    }
}

This is blocking to forward the HTTPS request to the downstream after the TLS termination. We might be able to change the scheme to HTTP to skip the check, but the better thing without touching the request would be introducing keep_original_uri flag not to touch the URI. This will help users to use the Hyper client more flexibly for various purpose.

if !self.config.keep_original_uri { // New Suggestion
    // CONNECT always sends authority-form, so check it first...
    if req.method() == Method::CONNECT {
        authority_form(req.uri_mut());
    } else if pooled.conn_info.is_proxied {
        absolute_form(req.uri_mut());
    } else {
        origin_form(req.uri_mut());
    }
}

If you are okay with this idea, I can raise a PR to this Hyper 0.14 and Hyper-Util package. Thanks in advance.