Select widget outputs None value as string

I have floppyforms ChoiceField rendering with a Select widget. The choices include a null value:

```
NUM_DOORS_2 = 2
NUM_DOORS_3 = 3
NUM_DOORS_CHOICES = (
(None, 'No. of doors'),
(NUM_DOORS_2, 2),
(NUM_DOORS_3, 3),
)

num_doors = forms.ChoiceField(NUM_DOORS_CHOICES, required=False)
```

When the select element renders the first option is:
`No. of doors`

This is different to Django's native forms which render the first option as:
`No. of doors`

The value returned by the floppyforms select list is the unicode string "None". The value returned by Django's native select list widget is None type.

This difference is significant and has produced a number of bugs in my project. I have fixed it by overriding the select widget template and checking for the string "None", but the None type is apparently being coerced into a string by the floppyforms select widget.

Is this behaviour intentional?

Reproduce the issue with the provided ChoiceField and compare floppyforms' Select widget output with Django's native widget. Trace the widget rendering path responsible for converting the null choice and verify that the rendered value and returned value match Django's empty-value behavior.

Written by the indexing model from the issue text.