Fix GET validation for lowercase http methods (#2497)

Some requests send method in lowercase (verified with curl and as a default for AWS ALB health check requests), but Go HTTP library constant MethodGet is in upper.

controllers/actions.summerwind.net/horizontal_runner_autoscaler_webhook.go

@@ -115,7 +115,7 @@ func (autoscaler *HorizontalRunnerAutoscalerGitHubWebhook) Handle(w http.Respons
 	}()
 
 	// respond ok to GET / e.g. for health check
-	if r.Method == http.MethodGet {
+	if strings.ToUpper(r.Method) == http.MethodGet {
 		ok = true
 		fmt.Fprintln(w, "webhook server is running")
 		return