Remove ConditionRenderer.refs, rewrite ConditionRenderer.update_for #49
Remove ConditionRenderer.refs, rewrite ConditionRenderer.update_for #49

update_for now checks the dumped form of the column's value for None;
this lets an attribute with value `set()` to be part of REMOVE even
though its loaded form is not None.

numberoverzero/bloop,numberoverzero/bloop