EmberJS - Computed headers value not called

In my Ember app I have subclassed RESTAdapter in order to dynamically update the authorisation headers for each request sent to my api:

import DS from 'ember-data';
import Ember from 'ember';

export default DS.RESTAdapter.extend({
    user: Ember.inject.service(),
    host: 'http://ift.tt/1FRtUsR',
    headers: Ember.computed(function() {
        this.get('user').getAuthHeaders();
    }),
    handleResponse(status, headers, payload) {
        if (headers['access-token']) {
            this.get('user').setToken(headers['access-token']);
        }
        return this._super(status, headers, payload);
    }
});

However, the computed function does not get called and I cannot work out why. Any help would be greatly appreciated!