Use the merged context when calling ExecuteTemplate

Fix a bug where global variables were missing.

httputil/renderer.go

@@ -45,7 +45,7 @@ func (r *Renderer) Render(w http.ResponseWriter, req *http.Request, templateName
 	// Render the template into a buffer, to prevent returning
 	// half-rendered templates when there is an error.
 	var buf bytes.Buffer
-	if err := r.tpl.ExecuteTemplate(&buf, templateName, data); err != nil {
+	if err := r.tpl.ExecuteTemplate(&buf, templateName, vars); err != nil {
 		log.Printf("template rendering error for %s: %v", req.URL.String(), err)
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return