Enable one entity namespace in multiple folders

With array_merge, only last namespace is used. In this case `%appDir%/model` would be overriden.

Use case for this fix:

**config.neon**
```yaml
doctrine:
	metadata:
		App:
			- %appDir%/model
```

**Extension.php**
```php
class Extension extends CompilerExtension implements IEntityProvider
{

	/**
	 * @return string[]
	 */
	public function getEntityMappings()
	{
		return ['App' => [__DIR__ . '/../model']];
	}

}
```