uclient-http: properly handle HTTP redirects via proxy connections

uclient-http: properly handle HTTP redirects via proxy connections

When uclient connects via an HTTP proxy connection we need to swap the
contexts .proxy_url and not the .url property (which is the connection
URL to the proxy server itself).

This fixes uclient erroring out in a redirect loop when connecting via
a proxy connection.

Ref: https://forum.openwrt.org/t/opkg-update-fails/25813
Signed-off-by: Jo-Philipp Wich <jo@mein.io>