Initial data problem when passed "0"

I have found that passing "0" as form_initial_data below line will return None

fields.py:29:

``` python
field_dict['initial'] = self.form_initial_data or self.field.initial
```

Maybe it should be some like this:

``` python
field_dict['initial'] = self.form_initial_data if self.form_initial_data is not None else self.field.initial
```
