Adds 'true' and 'false' to `NullBooleanField`.

JavaScript serializations of forms will sometimes render the boolean values as the strings 'true' and 'false', in lower case. Rather than repeat boilerplate in the JavaScript to circumvent this, it seems reasonable to allow Django to understand the lower-case versions of the booleans.

Related to [ticket 23129](https://code.djangoproject.com/ticket/23129)