Pass region code, not region object

Not sure how this ever worked, or why changing to Django 1.6 would have affected it.

Lower down this gets converted to a string using `str(...)`, and apparently that call used to extract the `ITreeRegion.code` field. But I see no method on `ITreeRegion` that would have accomplished that.

Fixes #1472
We may want to also rename this var to ``region_code`` to avoid confusion.