Fixed #6209: handle `BooleanField`s in `FormPreview` and `FormWizard`. In the...

Fixed #6209: handle `BooleanField`s in `FormPreview` and `FormWizard`. In the process, broke the the security hash calculation out to a helper function. Thanks to mcroydon and rajeshdhawan.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8597 bcc190cf-cafb-0310-a4f2-bffc1f526a37