Fix request.responseBody not defined

[mtscrb]

Related to #163

In nativeSend, the request.responseBody sometimes it's undefined, when the responseType is empty.

Refer to https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/responseType

We need to have a fallback to either responseText or response based on the responseType.
Something like

const response = new _Response.Response(request.status, request.responseBody || request.responseText || request.response, headers);

Also, in _handleResponse we are stringifying the response without checking the responseType
We should do something like:

      if (this.responseType === "blob") {
        if (body instanceof Blob) {
          this._response = body;
        } else {
          this._response = new Blob([body]);
        }
      } else if (this.responseType === "json") {
        this._response = JSON.stringify(body);
      } else {
        this._response = body;
      }