helper lookup issue in attributes

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName:'Ember Twiddle'
});

application.template.hbs

it works here: {{foo-bar}}<br>
but <a href={{foo-bar}}>look at my href</a>
<a href="{{foo-bar}}">or my href</a>
<a href="{{foo-bar}}/qux">or my href</a>

helpers.foo-bar.js

import Ember from 'ember';

export function fooBar(params/*, hash*/) {
  return "http://foobar.com/";
}

export default Ember.Helper.helper(fooBar);

twiddle.json

{
  "version": "0.4.13",
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "http://builds.emberjs.com/canary/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/1.13.13/ember-data.js",
    "ember-template-compiler": "http://builds.emberjs.com/canary/ember-template-compiler.js"
  }
}