title: Session already started in FOS User Bundle Registration Confirmation
body: <p>I am using FOSUserBundle for user registration. And when confirming a user with /register/confirm/xxxx-token-xxxxx I sometimes get the error "Failed to start the session: already started by PHP ($_SESSION is set)."</p>
  
  <p>This happens in the NativeSessionStorage::start() method. Obviously there is already a session ($_SESSION is an empty array in that case).</p>
  
  <p>This behaviour is very hard to reproduce. It works most of time. I am seeing this in dev and prod environment (both php 5.3.x)</p>
  
  <p>Any idea why $_SESSION is an empty array when initializing the session?</p>