Scenario:
Have a connection with DHCPv4 and a default-route. When externally
removing the default route (`ip route delete 0.0.0.0/0`) and issuing
`nmcli device reapply $IF`, the default route was not restored.
That was because when externally removing the default route,
we would remove the gateway from priv->con_ip4_config (see
update_ip4_config()). Later, when reapplying the connection,
the IP method doesn't actually change. So we would not restart
DHCP and thus there is no gateway around to add the default route.
The default route would only be restored after receiving a DHCP lease
in the far future.

Fix that, by always restarting the IP method.