Fix default values always being False for browsable API

This fixes a bug introduced in https://github.com/tomchristie/django-rest-framework/commit/28ff6fb1ec02b7a04c4a0db54885f3735b6dd43f that caused the default value for BooleanFields to be overridden when form data is passed to the serializer.  While #1248 already exists, and does fix the issue (somewhat, I'll post the issues there later), this fixes it at the source of the problem.

/cc @tomwalker