title: Devise AJAX - POST request : current_user is null
body: <p>I try to develop a single page application with authentication. I use Devise (Rails) and AngularJS.
  For some different reasons my rails application and my angularjs application are not on the same server. So, I have to handle cross domain issues... And I can't send X-CSRF-Token in my header.
  I can correctly  sign in and sign out and send GET request without problem. In my rails controllers, I can use current_user, it's correctly set.
  However, when I send a POST request, current_user is null. It seems my session_id is not sent. The problem is due to cross domain, because if I send my ajax request from the same server, it is OK.</p>
  
  <p>I think, I have 2 solutions : 
  - Don't use authentication cookie-based, but use token
  - Put front-end and back-end on the same server.</p>
  
  <p>Other ideas ? Why current_user is null when I send a POST request from cross domain ?</p>