You should be doing it as below,

let headers = new HttpHeaders().set('Content-Type', 'application/json')
                               .set('authorization', 'Bearer ' + token);

It is of type immutable Map so if you assign a new value it will reinitialize the object so you should be doing as

let headers = new HttpHeaders().set('Content-Type', 'application/json')
                               .set('authorization', 'Bearer ' + token);

or

let headers = new HttpHeaders().set('Content-Type', 'application/json');
headers = headers.set('authorization', 'Bearer ' + token);