Tests NULL values on ForeignKeys.

After this commit, the modified test should fail.
Revert "Replaces math.isnan with simple None comparisons."

This reverts commit ebbb44261bb1082c80f819b435fef91abb1795ce.

And this fixes the broken test introduced in the previous commit.
Merge pull request #25 from BertrandBordage/master

Fixes behaviour with NULL values on ForeignKeys when django-pandas is verbose.