docs: function setResponseHeaders(headers) {
              this.responseHeaders = {};
  
              for (var header in headers) {
                  if (headers.hasOwnProperty(header)) {
                      this.responseHeaders[header] = headers[header];
                  }
              }
  
              if (this.async) {
                  this.readyStateChange(FakeXMLHttpRequest.HEADERS_RECEIVED);
              } else {
                  this.readyState = FakeXMLHttpRequest.HEADERS_RECEIVED;
              }
          }