net/http: don't send IPv6 zone identifier in outbound request, per RFC 6874
When making a request to an IPv6 address with a zone identifier, for
exmaple [fe80::1%en0], RFC 6874 says HTTP clients must remove the zone
identifier "%en0" before writing the request for security reason.
This change removes any IPv6 zone identifer attached to URI in the Host
header field in requests.