async _fetchWithRequestBody(method, url, body) {
    const response = await this._fetch(this._normalizeURL(url).href, {
      method,
      body: JSON.stringify(body),
      headers: {...this._extraHeaders, 'content-type': 'application/json'},
    });

    return this._convertFetchResponseToReturnValue(response);
  }
@param {string} method
@param {string} url
@param {*} body
@return {Promise<any>}