url: Fix NO_PROXY env var to work properly with --proxy option.

The combination of --noproxy option and http_proxy env var works well
both for proxied hosts and non-proxied hosts.

However, when combining NO_PROXY env var with --proxy option,
non-proxied hosts are not reachable while proxied host is OK.

This patch allows us to access non-proxied hosts even if using NO_PROXY
env var with --proxy option.