title: How does symfony listen for the _switch_user?
body: <p>I have an application in which I have a superadmin role and various user roles with different privileges. I want to be able to impersonate as any of these users using the <code>_switch_user</code> query as shown on <a href="http://symfony.com/doc/current/book/security.html#impersonating-a-user">http://symfony.com/doc/current/book/security.html#impersonating-a-user</a></p>
  
  <p>However, when I add the query to the end of my url, it doesn't seem to do anything. I've played around with this for a long time now and can't seem to find a solution. I know that the user I'm logged in as has <code>ROLE_ALLOWED_TO_SWITCH</code>, but I can't quite seem to figure out how symfony does it.</p>
  
  <p>I'm using a custom authentication provider so I think it has something to do with that, but I'm not sure what I need to be looking at. I can post any code needed, but I'm really not sure what to post right now. </p>