this.http.get('https://some.com/questions.xml', {headers, responseType: 'text'})
  .mergeMap(
    res => xml2js.parseString(
      res,
      { explicitArray: false },
      (error, result) => {
        if (error) {
          throw new Error(error);
        } else {
          console.log(result);
          return result;
        }
      }
    )
  ).subscribe(
    data => { console.log("response", data); },
    error => { console.log(error); }
  );