title: How to reload helpers on rails mountable engine
body: <p>In my rails mountable engine:</p>
  
  <pre><code>config.to_prepare do
    # works fine, and reload automatically in development
    ApplicationController.helper :application
  
    # works fine, but doesn't reload. After restart server, it works.
    ApplicationController.helper Rails.application.helpers
  </code></pre>
  
  <p>It looks like fine when arg is symbol or string. But it doesn't work when arg is a module like <code>Rails.application.helpers</code>.</p>
  
  <p>Or is there a good way to get all helpers like <code>[:application, :users]</code> from <code>Rails.application.helpers</code>.</p>
  
  <p>Rails: 4.2.3</p>