Migrate from HttpModule to HttpClientModule. Angular 5

I am trying to migrate from angular HttpModule to angular HttpClientModule.

My current code working with HttpModule is as follows:

constructor(private _http: Http) { }

 token: IToken;
 errorMesage: string;

 private _candidateLoginUrl = 'http://myapi/v1/auth/login';

 login(userName: string, password: string): Observable<IToken> {

  let body = JSON.stringify({ username: userName, password: password });
  let headers = new Headers();
  headers.append("Content-Type", 'application/json');
  let options = new RequestOptions({ headers: headers });

  return this._http.post(this._candidateLoginUrl, body, options)
    .map((response: Response) => {

      this.token = <any>response.json().data;

      if (this.token && this.token.token) {            
        localStorage.setItem('currentUser', JSON.stringify(this.token));
      }

      return this.token;
    })
    .catch(this.handleError);
  } 

  private handleError(error: Response) {
    return Observable.throw('You are not authorized to get this resource');
  }

In order to keep the same logic I have done the following:

constructor(private _http: HttpClient) { }

token: IToken;
errorMesage: string;

private _candidateLoginUrl = 'http://myapi/v1/auth/login';

login(userName: string, password: string): Observable<IToken> {

  let body = JSON.stringify({ username: userName, password: password });  
  const headers = new HttpHeaders().set("Content-Type", 'application/json');

  this._http.post(this._candidateLoginUrl, body, { headers })    

  .map((response: Response) => {

            this.token = <any>response;

            if (this.token && this.token.token) {               
              localStorage.setItem('currentUser', JSON.stringify(this.token));
            }

            return this.token;
          })

  .catch(this.handleError);
}

private handleError(error: Response) {
  return Observable.throw('You are not authorized to get this resource');
}

However, I am getting an error from the login function: "a function whose declare type either 'void' not any must return value". What is the right way to migrate my code to HttpClientModule?