Thanks, Vajrasky. The v2 patch was almost correct.  What you couldn't know without being as deeply enmeshed in this code as I am is that the test failures from the encoders module were actually invalid.  We'd previously "fixed" them, but the fixes were incorrectly compensating for this bug in set_payload.  Once this bug was fixed, those "fixes" just needed to be backed out, a 'decode=True' added to their get_payload calls, and all the tests pass.

I *think* this is the last inconsistency in the model.  I hope.