docs: function(xhr, promise, headers, method) {
              // XDR cannot set requestheaders, only XHR:
              if (!xhr._isXDR) {
                  console.log(NAME, '_setHeaders');
                  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
                  ((method!=='POST') && (method!=='PUT')) || (headers && (CONTENT_TYPE in headers)) || xhr.setRequestHeader(CONTENT_TYPE, DEF_CONTENT_TYPE_POST);
              }
          }