A PointField with a None value raises a "ValueError: Cannot use object with type NoneType for a geometry lookup parameter" error when trying to invalidate the object.
Merge branch 'master' of git://github.com/clearcare/django-cacheops
Skip GIS tests for non-gis databases
Fix test pk race
Proper fix for null geometry fields

Initial fix in #100 works only for PointField and ignores invalidation
for it.