Add test exposing UnitOfWork merge bug

I hope that I'm doing something wrong and that this is not a bug ...
Note that the following works:

```php
$country = new MergeCompositeToOneKeyCountry();
$country->country = 'US';
$state = new MergeCompositeToOneKeyState();
$state->state = 'CA';
$state->country = $country;

$newCountry = $this->_em->merge($country);
$state->country = $newCountry;
$this->_em->merge($state);
```