If no pattern matches a request, but a pattern would have
matched if the request had a different method, then
serve a 405 (Method Not Allowed), and populate the
"Allow" header with the methods that would have succeeded.
Updates #61640.
Change-Id: I0ae9eb95e62c71ff7766a03043525a97099ac1bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/528401 Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>