Make RewriteConds for `x → www.x` more permissive

The benefit of this change is that it makes the `example.com` →
`www.example.com` rewrite work properly not only for `127.0.0.1`
and `localhost`, but also with all different virtual names for
the local environment.

Thanks: Chen Hua (@a25ce1).

Close h5bp/server-configs-apache#11.