login(username: string, password: string): Observable<User> {

     const body = { username: username, password: password };
     const headers = new HttpHeaders();
     headers.append("Authorization", "Basic " + btoa("username:password"));
     headers.append("Content-Type", "application/x-www-form-urlencoded");

     return this.http
               .post<User>(`${BASE_URL}/sign-in`, body, { headers: headers })
               .do(user => (this.user = user));
}