title: mountable engine + capybara (with rspec) + main_app.helpers
body: <p>In my <code>Admin::ApplicationController</code> (which is a mountable engine) , I include main_app's helepr methods like this: <code>helper ::ApplicationHelper</code> .</p>
  
  <p>But when I run capybara + rspec tests, I get a error like this: <code>undefined method 'foo' for #&lt;#&lt;Class:0x007fe9c14821e8&gt;:0x007fe9c1485aa0&gt;</code></p>
  
  <p>How can I fix this issue? Thanks on advance!</p>