Revert "Replaces math.isnan with simple None comparisons."
Revert "Replaces math.isnan with simple None comparisons."

This reverts commit ebbb44261bb1082c80f819b435fef91abb1795ce.

And this fixes the broken test introduced in the previous commit.

chrisdev/django-pandas,sternb0t/django-pandas,arcticshores/django-pandas,perpetua1/django-pandas