This fixes a bug that was introduced in 28ff6fb [1] for the
browsable API, specifically with how it handled default values
for boolean fields. Previously, it had a global default for
boolean fields set to `False`, which was different than the
standard None that was used elsewhere. Because this only needed
to be done for the browsable API, a fix was put into place that
only set the default to `False` when form data was passed into
the serializer. This had the unintended side effect of overriding
any default set on the boolean field.
This fixes#1101 [2] by only overriding the default if the default is
`None`, which is the default for all fields.
[1]: 28ff6fb1ec
[2]: https://github.com/tomchristie/django-rest-framework/issues/1101
When getting the help_text from a field where `many=True`, Django
appends 'Hold down "Control", or "Command" on a Mac, to select more than
one.' to the help_text. This makes some sense in Django's ModelForms,
but no sense in the API.
