Pass request headers to native xhr request in xhrinterceptor

[zzarcon]

Currently, when there is no registered route handler we bypass the request but we are not passing the headers on the XHR case. That can be solved by setting the request headers into the real request:

// xhrinterceptor

nativeSend(data?: any): void {
    const request = new NativeXMLHttpRequest();

    request.timeout = this.timeout;
    request.onload = () => {
      const headers = request
        .getAllResponseHeaders()
        .split("\r\n")
        .reduce((previous, current) => {
          const [header, value] = current.split(": ");
          return {
            ...previous,
            [header]: value
          };
        }, {});
      const response = new KakapoResponse(
        request.status,
        request.responseBody,
        headers
      );
      this._handleResponse(response);
    };

    request.open(this._method, this._url);
    // start of the fix
   Object.keys(this._requestHeaders).forEach(headerName => request.setRequestHeader(headerName, this._requestHeaders[headerName]))
   // end of the fix
    request.send(data);
  }
}

cc @mtscrb