Validator: Fix casting record_specific_diffs object to list for json serialization

[barbarahui]

I'm a bit confused as to what exactly is going on here in terms of what type of object record_specific_diffs is (prior to any attempts to set its type to list). In case of collection 27566, it seems be an instance of dict. The set_encoder function clearly expects it to be of type set or SetOrdered, but I needed to add dict as an option.

The other problem was that calling set_encoder() as part of the json.dumps call (by adding the default=set_encoder argument) results in the obj being of type type (the type superclass). Calling set_encoder separately fixes this problem.

In reading through the code, it seems to me that record_specific_diffs is always a dict. However, if we don't explicitly cast it to a list by calling set_encoder on it, then calling json.dumps on it results in a "TypeError: Object of type type is not JSON serializable" error. So it is somehow both a dict and a type object? I'm deeply confused.

If the changes I've made here make sense, then we should probably change the name of the set_encoder function. All I can think of is diffs_obj_to_list.