EmbedOne with discriminator field/map.

Fixed a bug when using EmbedOne with discriminator field/map eg.

```php
/**
 * @ODM\EmbedOne(
 *      discriminatorField="type",
 *      discriminatorMap={
 *         "RecordLabel"    = "Zoop\Shard\Test\ODMCore\TestAsset\Document\RecordLabel",
 *         "Artist"         = "Zoop\Shard\Test\ODMCore\TestAsset\Document\Artist"
 *      }
 * )
 */
protected $license;
```