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.

Fixes #9544.

Change-Id: I7406bd0aa961d260d96f1f887c2e45854e921452
Reviewed-on: https://go-review.googlesource.com/3111Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>