* Fix `Gson.getDelegateAdapter` not working properly for `JsonAdapter`
* Address review feedback and add comments regarding thread-safety
* Revert InstanceCreator instance validation
* Disallow `null` as `skipPast`
* Avoid `equals` usage in `getDelegateAdapter` & minor other changes
Previously `getDelegateAdapter` called `factories.contains(skipPast)`,
but unlike the other comparisons which check for reference equality,
that would have used the `equals` method.
This could lead to spurious "GSON cannot serialize ..." exceptions
if two factory instances compared equal, but the one provided as
`skipPast` had not been registered yet.
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