Fix for #955

If a ToOneField/ToManyField has a callable attribute that returns None, We get the issue #955 . I have sent a small fix and included a test case inside tests/core/fields.py - test_dehydrate_with_callable with field_3 that verifies an APIError return value(As per the fix). The message returned may not be the right one though. Please review and let me know if any more changes are required.