Fixed #7753: clean `NullBooleanField` correctly when using `HiddenInput`....

    widget = NullBooleanSelect

    def clean(self, value):

        return {True: True, False: False}.get(value, None)

        """

        Explicitly checks for the string 'True' and 'False', which is what a

        hidden field will submit for True and False. Unlike the

        Booleanfield we also need to check for True, because we are not using

        the bool() function

        """

        if value in (True, 'True'):

            return True

        elif value in (False, 'False'):

            return False

        else:

            return None

class ChoiceField(Field):

    widget = Select

>>> f.clean('3')

>>> f.clean('hello')

# Make sure that the internal value is preserved if using HiddenInput (#7753)

>>> class HiddenNullBooleanForm(Form):

...     hidden_nullbool1 = NullBooleanField(widget=HiddenInput, initial=True)

...     hidden_nullbool2 = NullBooleanField(widget=HiddenInput, initial=False)

>>> f = HiddenNullBooleanForm()

>>> print f

<input type="hidden" name="hidden_nullbool1" value="True" id="id_hidden_nullbool1" /><input type="hidden" name="hidden_nullbool2" value="False" id="id_hidden_nullbool2" />

>>> f = HiddenNullBooleanForm({ 'hidden_nullbool1': 'True', 'hidden_nullbool2': 'False' })

>>> f.full_clean()

>>> f.cleaned_data['hidden_nullbool1']

True

>>> f.cleaned_data['hidden_nullbool2']

False

# MultipleChoiceField #########################################################

>>> f = MultipleChoiceField(choices=[('1', 'One'), ('2', 'Two')])