Update cookbook/security/entity_provider.rst

Also serialize the username, to prevent issues with reloading the user (i.e. losing ROLE_PREVIOUS_ADMIN after you switched between accounts). See http://12wiki.blogspot.nl/2012/09/symfony-2-and-case-of-missing.html for more details

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all (or 2.1+)
| Fixed tickets | none