Fix bug where autosave interprets all checkbox inputs as checked
Fixes to AdminAutoSaveMixin.get_form()

- ModelAdmin.get_form() passes kwargs.get('form', self.form) on to
  the modelform_factory. If we want to extend the form class returned
  from get_form() we will have better luck by passing the 'form' kwarg
  than extending the result of the ModelForm factory (or, for that matter,
  performing a deepcopy on it).
- Adds method full_clean() to IllegalForm to prevent the call to the model
  and form clean methods, as these can have side-effects. Within the extended
  method we clear out any validation errors and delete self.cleaned_data, to
  mimic these lines in the parent:

    if self.errors:
        del self.cleaned_data

  The deletion of self.cleaned_data in ModelForm.full_clean() in the case of
  ValidationErrors goes away in django 1.6. This shouldn't matter in our case
  since we want to prevent field validation anyway.