fixed prepareQueryElement() "set" strategy issue

After this change the `set` strategy allows you to query for a value in a specific embedded document.

### Before

```
$userRepo->findOneBy(array('profiles.facebook.id' => 1234));
-->
db.users.findOne({ "profiles.facebook": 1234 }) // what happened to ".id"?
```

### After

```
$userRepo->findOneBy(array('profiles.facebook.id' => 1234));
-->
db.users.findOne({ "profiles.facebook.id": 1234 })
```