Allow passing a var to :access-control-allow-origin

[dwwoelfel]

This allows you to pass a var as :access-control-allow-origin. It's nice in development because you can make changes to the function without having to restart the server.

Example:

(defn allow-origin? [req]
  (= (:uri req) "/my-cors-path"))
  
(wrap-cors handler :access-control-allow-origin #'allow-origin?)