Avoid warning if array is defined for settings in metadata

If metadata contains an Array like
```php
  array('group' => 'foo', 'name' => 'bar',       'type' => 'aarr',    'value' => array('foo' => 'bar','baz' => 'foo'))
````
 in settings an warning will be shown:
> Warning: unserialize() expects parameter 1 to be string, array given

so here a quick fix to avoid this warning.