[Security] User refreshment from identical users provider type

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

When we are using two privoders with the same supported class but with different users, there is a faill because the first `UsernameNotFoundException` will stop the refresh processus.
Where do you see this line breaks CI? I don't see any break there https://travis-ci.org/symfony/symfony/jobs/41113847 :/
Previously, if you hadn't got any userProvider registered there, , the code shoud not throw an error : https://github.com/symfony/symfony/blob/2.7/src/Symfony/Component/Security/Http/Firewall/ContextListener.php#L173.
Previously the code return null.