Fix csv export 

This fixes the csv export when a related model does not exist.

example: 
```
Store:
  fields:
    default: id, name, country.name, state.name, city.name, zip_code, street
```
This currently breaks if country,state or city is nil.

This fix should/could probably be applied in the stable branches as well.
Merge pull request #462 from tkoenig/patch-2

Fix csv export