[5.7] Fix QueueableCollection serialization of Eloquent Models when using Binary IDs

[tonysm]

Found this issue during development where we have a model that uses binary UUIDs. While the single model serialization works fine, the collection serialization + JSON encoding breaks (UTF-8 issues).

Added a test demonstrating the problem we faced (the testJsonSerializationOfCollectionQueueableIdsWorks one) and some other testing that still works for Pivot Models as well as Eloquent Models.

[taylorotwell]

I don't understand this change. We were checking for Pivot and now we're checking for something that Pivot doesn't even implement?

[tonysm]

Pivot extends from Model, which implements that interface. However, Pivot overrides the behavior of the base Model for this particular method in the asPivot trait.

As the Eloquent Collection's getQueueableIds method is the implementation of QueueableCollection, I thought it would make sense to have it relying on the QueueableEntity (where the getQueueableId method is defined) instead of it relying in the Pivot class specifically.

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The change makes sure the Collection::getQueueableIds() return the Item::getQueueableId() for anything that implements the QueueableEntity contract (as the base Model implements), not specifically the Pivot class.