Steps to Reproduce

1. run the attached zul with developer tool open > network
2. when the confirm dialog (the server is temporarily out of service) pops up, click ok

Current Result

zk AU get 467 response

Expected Result

the response should be 200

Debug Information

• it sends an object to the server instead of a string: reqInf.content

    function ajaxSendNow(reqInf) {
      // eslint-disable-next-line no-undef
      var fetchOpts = {
        credentials: 'same-origin',
        method: 'POST',
        headers: {
          'Content-Type': zAu.ajaxSettings.contentType,
          'ZK-SID': '' + reqInf.sid
        },
        body: reqInf.content
      };
      ....
      zAu._fetch(reqInf.uri, fetchOpts).then(function (response) {