public getUserData(username: string, password: string) {

    const httpOptions = {
      headers: new HttpHeaders({
        'Content-Type':  'application/json',
      })
    };

    const user = new UserDTO();
    user.username = username;
    user.password = password;
    this.http.post<UserDTO>('http://localhost:8080/login', user, {observe: 'response', headers: httpOptions.headers}).subscribe(res => {
      console.log(res.headers.get('Authorization'));
    });
  }