code: function(xhr, promise, headers, method) {
              // XDR cannot set requestheaders, only XHR:
              if (!xhr._isXDR) {
                  var name;
                  if ((method!=="POST") && (method!=="PUT")) {
                      // force GET-request to make a request instead of using cache (like IE does):
                      headers["If-Modified-Since"] = "Wed, 15 Nov 1995 01:00:00 GMT";
                      // header "Content-Type" should only be set with POST or PUT requests:
                      delete headers[CONTENT_TYPE];
                  }
                  // set all headers
                  for (name in headers) {
                      xhr.setRequestHeader(name, headers[name]);
                  }
  
                  // in case of POST or PUT method: always make sure "Content-Type" is specified
                  !BODY_METHODS[method] || (headers && (CONTENT_TYPE in headers)) || xhr.setRequestHeader(CONTENT_TYPE, DEF_CONTENT_TYPE_POST);
              }
          }
docs: Adds the `headers`-object to `xhr`-headers.
  
  @method _setHeaders
  @param xhr {Object} containing the xhr-instance
  @param headers {Object} containing all headers
  @param method {String} the request-method used
  @private