portalHelper: Don't go through proxies for captive portals

portalHelper: Don't go through proxies for captive portals

When using an SSH tunnel (through a SOCKS proxy) to funnel all
the outgoing traffic, we need the captive portal to not go through that
proxy, otherwise we can't go through the proxy because we're not
connected to the Internet and we can't go through the portal because
we're not connected through the proxy.

This fixes a blank captive portal window and no error reporting in that
particular configuration.

https://bugzilla.gnome.org/show_bug.cgi?id=769692