This fixes a bug that was introduced in 28ff6fb1 [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]: https://github.com/tomchristie/django-rest-framework/commit/28ff6fb1ec02b7a04c4a0db54885f3735b6dd43f
[2]: https://github.com/tomchristie/django-rest-framework/issues/1101