native-fetcher: bypass the resolver for ipv4 addresses

Bypass the resolver for ipv4 addresses

ipv6 support would be nice too, but would require other code to be
updated to that as well in this file. I created issue
https://github.com/pagespeed/ngx_pagespeed/issues/354 for that

This fixes https://github.com/pagespeed/ngx_pagespeed/issues/351