[Fail Case] Add test for aggregator transforms

[hailwood]

@nunomaduro this is the failing test for #158

You'll note that we've setup an extended "Thread" model where the toSearchableArray method specifies to only use the date transform.

The testTransformsAreRespectedOnSingleModel passes, I've only added it to ensure this is tested in the future.

The testModelTransformsAreRespectedOnAggregators test then fails, as while the following check passes on the single model, it fails on the aggregate as the '100' has been cast to 100.

return $argument[0]['subscriber_count'] === '100';

The test can be made to pass by adding the following function to our ThreadAggregatorUsingOnlyDateTransform class as this causes the check at https://github.com/algolia/scout-extended/blob/master/src/Jobs/UpdateJob.php#L113 to pass and as such we don't call the default transformers again, however I feel a better fix would be an update in the hasToSearchableArray method on UpdateJobs`.

public function toSearchableArray(): array
{
  return parent::toSearchableArray();
}

P.S. sorry about the non-matching code style. I just quickly wrote this up to make the point clear.