Why is the value of a Checkbox forced to None?

The `_format_value` of a `CheckBoxInput` returns `None` if value is valid (`True` or `False`) .

See [Line 446](https://github.com/gregmuellegger/django-floppyforms/blob/master/floppyforms/widgets.py#L446)
```
def _format_value(self, value):
if value in ('', True, False, None):
value = None
else:
value = force_text(value)
return value
```

Why don't keep the value as it is rather than returning `None`? I think this cause a different behavior from other field types.

In my case, it caused a bug when upgrading an application and it was very difficult to find.

Thanks for explaining the reason.

Best
luc

Read floppyforms/widgets.py at line 446 and trace how CheckBoxInput._format_value handles True, False, empty strings, and None. Compare this behavior with other field types and determine the expected checkbox value handling; the issue is complete when the behavior or its rationale is clarified and the reported upgrade difference is addressed.

Written by the indexing model from the issue text.