[GH-554] Fixed ObjectHydrator when namespace alias is given.

This issue is created automatically through a Github pull request on behalf of beregond:

Url: https://github.com/doctrine/doctrine2/pull/554

Message:

Fixes problem when executing native query:
```php
$rsm = new ResultSetMapping();

$rsm->addEntityResult('DbBundle:Player', 'p');
$rsm->addFieldResult('p', 'id', 'id');
$rsm->addFieldResult('p', 'name', 'name');

$rsm->addJoinedEntityResult('DbBundle:User', 'u', 'p', 'user');
$rsm->addFieldResult('u', 'user_id', 'id');

$em->createNativeQuery('...', $rsm);
```

Hydrator couldn't find cached metadata, when "joined entity result" class name wasn't fully qualified name (as in the example above).