[release-branch.go1.23] internal/godebugs: fix old value for httpservecontentkeepheaders

[release-branch.go1.23] internal/godebugs: fix old value for httpservecontentkeepheaders

The pre-Go 1.23 behavior is httpservecontentkeepheaders=1.

For #66343

Change-Id: If6f92853b38522f19a8908ff11ac49b12f3dc3e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/593775
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/593795
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>