Can´t retrieve access token in login response headers

[JulianSansat]

I'm implementing a ionic 2 client to use the rails api, already got the user´s registration working, but the problem is the login, the rails app sends the "OK" response with the headers, but I´m having trouble to retrieve the access-token from the headers because the json is all nested in a strange way, and I didn´t override nothing, is the default devise token auth response.
https://i.stack.imgur.com/WtfXv.png
http://stackoverflow.com/questions/43714404/retrieve-access-token-from-devise-auth-response-headers

login(credentials) {
    return new Promise((resolve, reject) => {
        let headers = new Headers();
        headers.append('Content-Type', 'application/json');
        this.http.post(apiUrl+'auth/sign_in', JSON.stringify(credentials), {headers: headers})
          .subscribe(res => {
            resolve(res.json());
          }, (err) => {
            console.log(err);
          });
    });
  }

the response is getting null or undefined. Viewing on the print the way the response is nested could be the reason that the function can´t reach de access-token value.

I´m wonder what i´m doing wrong, because with the Angular2-token (which is not compatible with ionic) it doesn´t need to get the values from a nested json.
Thanks

[JulianSansat]

I figured out already, it was sending the response as a json, so on the other component i wasn´t able to use de http methods, so i used res.headers.get('access-token') with de http lib of angular, and did the trick. I´m implementing an ionic client because, there is no suport yet for an ionic mobile solution with devise token auth, I hope I can implement the rest of the features and share the project.