User_saml fix for OC6

Fixes broken user creation and logout on OC6.

```createUser``` needs to be called inside ```checkPassword``` function, since it never gets called in post_login hook. 
Once backend's ```checkPassword``` returns user $uid, user gets cached by ```getUserObject``` call: 
``` 
/* private/user/manager.php (checkPassword) */
$uid = $backend->checkPassword($loginname, $password);
if ($uid !== false) {
    return $this->getUserObject($uid, $backend);
```
```
protected function getUserObject($uid, $backend) {
   $this->cachedUsers[$uid] = new User($uid, $backend, $this, $this->config);
```
This results in ```userExists``` returning true because it finds user in cachedUsers. So the following in the post_login hook will never get called:
```
  if (!OC_User::userExists($uid)) {
    	OC_User::createUser($uid, $random_password);
```