Added support for validation on field attribute with type list

I found that when trying to validate a field attribute with type list I get this error:
```
{
    "_status": "ERR",
    "_issues": {
        "music_type": "value '[u'elem1', u'elem2']' must exist in resource 'types', field 'key'."
    },
    "_error": {
        "message": "Insertion failure: 1 document(s) contain(s) error(s)",
        "code": 400
    }
}
```
I've added support for this type of field attribute.