Fix for the location filter restriction

Is `self.request.user` ever not a string? If it is, do you want to convert `None` or whatever other object it could be to a string and look that up as a username? (probably not) If you're worried about catching the `None` case, then you should save a database call and explicitly check for `None`:

```python
if self.request.user is None:
    user = None
else:
    user = CouchUser.get_by_username(self.request.user)
```

If `self.request.user` is ever not a string and the string representation of it could be a valid username, then should use `unicode(...)` rather than `str(...)` here.