[release-branch.go1.15] net/http/httputil: always remove hop-by-hop headers

[release-branch.go1.15] net/http/httputil: always remove hop-by-hop headers

Previously, we'd fail to remove the Connection header from a request
like this:

    Connection:
    Connection: x-header

Updates #46313
Fixes #46314
Fixes CVE-2021-33197

Change-Id: Ie3009e926ceecfa86dfa6bcc6fe14ff01086be7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/321929
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Trust: Katie Hockman <katie@golang.org>
Trust: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/323091
Run-TryBot: Katie Hockman <katie@golang.org>