CORS middleware for Joy framework

cors.janet

(defn cors [handler &opt opts]
  (default opts @{})
  (def defaults @{"Access-Control-Allow-Origin" "*"
                  "Access-Control-Allow-Methods" "GET, OPTIONS"
                  "Access-Control-Allow-Headers" "Content-Type"
                  "Access-Control-Max-Age 86400})
  (def options (merge defaults opts))
  (fn [request]
    (if (= "OPTIONS" (get request :method))
      @{:status 204, :body "" :headers options}
      (let [response (handler request)]
        (when response
          (update response :headers merge options))))))