Fix ipv6 support for http. When URL contain domain, not IP address.

Before this patch socket created before domain was resolved, and it
always create with AF_INET (ipv4), but we must create socket only after
domain was resolved to understad which protocol family have domain
address.

Thank to Patrick Pelletier, who found this bug.

Mailing list:
http://www.mail-archive.com/libevent-users@seul.org/msg01718.html
http://archives.seul.org/libevent/users/Nov-2012/msg00003.html