fixed UserProvider::refreshUser() not checking the class

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | (not sure)
| Deprecations? | no
| Tests pass?   | Travis?
| License       | MIT

This PR fixes an issue that the user-provider always refreshes the user if the `User` class or Propel-model is given, but the class inheritance is actually not met.

If you have two user-classes which both extend the `FosUserBundle\Model\User` it will refresh the wrong user.