Fixed #113 - a unique vaildation error might be thrown if a to-be created instance exists.

```python
class LonelyPersonWithPostSaveHook(models.Model):
    only_friend = models.OneToOneField(User) 

def lonely_user_with_post_save_hook(sender, instance, created, **kwargs):
    """Create a person when a new user account is created"""
    if created:
        lp = LonelyPersonWithPostSaveHook()
        lp.only_friend = instance
        lp.save()

post_save.connect(lonely_user_with_post_save_hook, sender=User)

mommy.make(LonelyPersonWithPostSaveHook)     # used to throw unique validation error in Mommy.instance()
```

This pull request fixes this behaviour (tests included).