IdentityInterface::findIdentity should return active users

```
interface IdentityInterface
{
    /**
     * Finds an identity by the given ID.
     * @param string|integer $id the ID to be looked for
     * @return IdentityInterface the identity object that matches the given ID.
     * Null should be returned if such an identity cannot be found
     * or the identity is not in an active state (disabled, deleted, etc.)
     */
    public static function findIdentity($id);
```
Merge pull request #4216 from SDKiller/patch-3

IdentityInterface::findIdentity should return active users