Fix infinite loop that occurs when an `data\Entity` class is used as `data\Entity::_model`.

This may bug occurs when using external ORM which are not based on core `data\Model`.
I'm not sure for this one because at first glance it looks like `data\Entity::_model` shouldn't be filled with something which is not a core `data\Model` subclass.
However if it makes sense, maybe a:
```php
!is_a($model, 'lithium\data\Entity', true)
```
would here be better than a: 
```php
method_exists($model, '_object')
```
for fixing this issue ?