Raise error when model with empty pk field is passed

[ffigiel]

Automated tests in my project failed because it turned out that bulk_update wasn't creating the new model instances I had in the list. I think bulk_update should raise a ValueError saying that it can operate on existing rows only.

[ffigiel]

Looks like #66 is another good take at this.

[jonathan-s]

If you start using that PR, just beware that it might introduce sql injection vulnerability atm. The values aren't properly escaped. I need to fix that.

[ffigiel]

It's fine, I updated my code to build two lists, one for bulk_update and other for bulk_create. Thanks for the info, this vulnerability stinks.