Tests for taggit add_test(), shows failure addressed by cfpb/collab#16
Prevent error tag_category is 'None', which is a legal value for the field
Merge pull request #17 from m3brown/tag_fields_test

Tests for taggit add_test(), shows failure addressed by cfpb/collab#16
Merge pull request #16 from m3brown/tag_fields

Prevent error if content_type or tag_category are 'None'