VALIDATE_FILTERS setting doesn't work for filter on sub-document field

[lmoretto]

Activating the VALIDATE_FILTERS setting causes request with filters on sub-documents' fields to fail.

Steps to reproduce
Suppose you have a domain resource with the following schema:

'collection': {
    'schema': {
        'sub_documents': {
            'type': 'list',
            'schema': {
                'type': 'dict',
                'schema' : {
                    'foo': {'type': 'string'}
                    'bar': {'type': 'integer'}
                }
            }
        }
    }
}

Performing a request like:
http://<api_server>/collection?where={"sub_documents.bar": 5}
returns the following response:

{
    "_status": "ERR",
    "_error": {
        "code": 400,
        "message": "filter on '%s' is invalid"
    }
}

This seems to be caused in eve/utils.py at lines 403-406:

if config.VALIDATE_FILTERS:
    res_schema = config.DOMAIN[resource]['schema']
    if key not in res_schema:
        return "filter on '%s' is invalid"

In this scenario, key will be sub_documents.bar which is not a valid key in the res_schema dict (whereas sub_documents is).

[nicolaiarocci]

Excellent. Looks like we're missing support for sub-document parsing here. Feel free to submit a PR if so inclined, I will gladly review it.