docs: function _setResponseHeaders(headers) {
      this.responseHeaders = {};
  
      for (var header in headers) {
        if (headers.hasOwnProperty(header)) {
            this.responseHeaders[header] = headers[header];
        }
      }
  
      if (this.forceMimeType) {
        this.responseHeaders['Content-Type'] = this.forceMimeType;
      }
  
      if (this.async) {
        this._readyStateChange(FakeXMLHttpRequest.HEADERS_RECEIVED);
      } else {
        this.readyState = FakeXMLHttpRequest.HEADERS_RECEIVED;
      }
    }