ShortUrls cannot be retrieved with latest MongoDB driver

This PR fixes a bug where the latest MongoDB driver would return no fields from a `findOne()`-call with an invalid field specification.

This currently returns all fields except `_id`. If we want to explicitly return the fields that were specified earlier, it could be changed to:
```php
array(
    '_id' => false,
    'publicKey' => true,
    'imageIdentifier' => true,
    'extension' => true,
    'query' => true,
)
```