This matches the net/http API.
Updates #59473.
Change-Id: I99917cef3ed42a0b4a2b39230b492be00da8bbfd
Reviewed-on: https://go-review.googlesource.com/c/go/+/548355
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
