docs: function getResponseHeader(header) {
              if (this.readyState < IsotropyXMLHttpRequest.HEADERS_RECEIVED) {
                  return null;
              }
  
              if (/^Set-Cookie2?$/i.test(header)) {
                  return null;
              }
  
              header = header.toLowerCase();
  
              for (var h in this.responseHeaders) {
                  if (h.toLowerCase() == header) {
                      return this.responseHeaders[h];
                  }
              }
  
              return null;
          }