go.net/proxy: fix IP address determination
go.net/proxy: fix IP address determination

net.IP represents either an IPv4 or IPv6 address. In case of
the address of IPv4 node, it uses IPv4-mapped IPv6 address
format by default.

R=agl, dave
CC=golang-dev
http://codereview.appspot.com/6782078

gigq/go.net,fernandez14/go.net,polydawn-ports/go.net,andrewzeneski/go.net,ssor/go.net,go-mirrors/go.net