See #18508

This commit adds http Client support for socks5 proxies.

Change-Id: Ib015f3819801da13781d5acdd780149ae1f5857b
Reviewed-on: https://go-review.googlesource.com/35488Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>