Figure Out "unpacking" in Aggregations Endpoint

[nodanaonlyzuul]

In #45 we discovered aggregations endpoints
were giving bad responses. (See that issue for details).

We used git bisect (plug plug) to find out the breaking change was 482af03.
Which led me to submit a fix in #46.

Sooooo....

Was I right to re-add serializationOpts?
Was the thinking that the AggregationsSerializer wouldn't use the :packed_fields anyway because
and would use unpacked fields?

If that's the case then we should be able to:

1. Remove my options finagling.
2. Watch the integration test fail.
3. Make the appropriate changes in the AggregationsSerializer.
4. Watch the integration test pass again.
5. Add a unit test for AggregationsSerializer?

Orrrr....

Should we just keep the code introduced / re-added in #46.

CCing: @jobinthomasnypl