corehttp: Gateway handler: add Allow headers when returning MethodNot…

…Allowed

Spec says that response with 405 must set Allow headers.


This commit was moved from ipfs/kubo@7340543

gateway/core/corehttp/gateway_handler.go

@@ -127,6 +127,9 @@ func (i *gatewayHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	if !i.config.Writable {
 		status = http.StatusMethodNotAllowed
 		errmsg = errmsg + "read only access"
+		w.Header().Add("Allow", http.MethodGet)
+		w.Header().Add("Allow", http.MethodHead)
+		w.Header().Add("Allow", http.MethodOptions)
 	} else {
 		status = http.StatusBadRequest
 		errmsg = errmsg + "bad request for " + r.URL.Path