proxy: use the same data for CONNECT and Host header

[nikstur]

The CONNECT and Host header should both point to the proxied host that we want to eventually reach, not to the proxy itself.

I'm no expert on proxies but MDN suggest that these headers should use the same data: https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/CONNECT

Also I used to be able to send proxied requests via socat to an Nginx with ureq < 3.0.0 and now it doesn't work anymore. socat now forwards the HTTP request as is to Nginx (telling me it doesn't recognize it as a valid proxy request) and Nginx replies with a 400. In previous ureq versions the CONNECT and Host headers used the same data: https://github.com/algesten/ureq/blob/2.12.1/src/proxy.rs#L187

[algesten]

Thanks!

This was a last minute change I did thinking "it must surely be the proxy host". And I realise now I did no googling on that.