Rewrite proxied Host header when URI comes in the form of absolut-URI

[dmpetroff]

https://www.rfc-editor.org/rfc/rfc9112.html#section-3.2.2

When a proxy receives a request with an absolute-form of request-target, the proxy MUST ignore the received Host header field (if any) and instead replace it with the host information of the request-target. A proxy that forwards such a request MUST generate a new Host field value based on the received request-target rather than forward the received Host field value.

Currently we only act as origin-server ignoring Host header in presence of absolute-URI

[voodam]

It was decided on meeting that this issue should be done after some another @const-t's issue, so I set him as assignee.

[const-t]

When making a request to a proxy, other than a CONNECT or server-wide OPTIONS request (as detailed below), a client MUST send the target URI in "absolute-form" as the request-target.
...
When a proxy receives a request with an absolute-form of request-target, the proxy MUST ignore the received Host header field (if any) and instead replace it with the host information of the request-target. A proxy that forwards such a request MUST generate a new Host field value based on the received request-target rather than forward the received Host field value.

This is a requirement for requests to a forward-proxy, where the client initially plans to send his request through a proxy. In this case, the forward proxy takes the scheme and address from the uri and connects using them, even over HTTP1.0 protocol. We are a reverse proxy and when the client makes a request to us, he initially does not want to proxy his request, he wants to receive a response as from the origin server. And therefore we must behave as the origin server.