use "_all" instead of "q" for filtering

This occured when using django-esutils on string fields, combined with
other terms.

e.g.:

This query was returning no record:

```
curl http://10.10.10.10:9201/app/index_name/_search -d \
   '{"filter": {"and": [{"term": {"item_id": 1}}, {"term": {"q": "stuff"}}]}}'
```

This one was returning all corresponding records:

```
curl http://10.10.10.10:9201/app/index_name/_search -d \
    '{"filter": {"and": [{"term": {"item_id": 1}}, {"term": {"_all": "stuff"}}]}}'
```