Update_content_binding and update_content_binding_subtype appear to make loaddata fail

[MarkDavidson]

Currently, I'm having a hard time doing something like:
python manage.py dumpdata taxii_services > data.json
then
python manage.py loaddata data.json
because I get the following error:
Could not load taxii_services.ContentBindingSubtype(pk=1): columns content_binding_id, subtype_id are not unique

I'm thinking the root cause might be because the post_save hooks for ContentBinding and ContentBindingSubtype do not properly account for the fact that the ContentBindingAndSubtype might already exist.

[MarkDavidson]

... and now I'm having trouble replicating the issue.

I'm thinking that changing the functions to use get_or_create would remediate the issue. However, I can't replicate the issue so I'm not going to fix it right now.