Fix DateTime Timestamps when inserting a new Model

Eloquent's Timestamp-Getters omitted to return a value when dealing with DateTime Objects.

This produced empty Timestamp Attributes  when dealing with the $model instances returned after inserting a new record:
```php
$user = array('username' => 'test', 'email' => 'test@test.com');
$created = User::create($user);

// returns NULL / false
var_dump($created->created_at):
// returns NULL / false
var_dump($created->updated_at): 
```
Full example here: http://paste.laravel.com/eQ5


This PR fixes the issue and adds a new test.