A little function that accepts a url, and returns a url that matches the current protocol.

match_protocol.js

window.match_protocol = function(url) {
  var RE_PROTO = /^\w+:\/\//;
  if(RE_PROTO.test(url)) {
    var protocol = window.location.protocol;
    url = url.replace(RE_PROTO, protocol + "//");
  }
  return url;
};