* Document how `JsonAdapter` creates adapter instances & add tests
* Extend `JsonAdapter.nullSafe()` documentation
* Improve test for JsonAdapter factory returning null
Existing test `JsonAdapterNullSafeTest` had misleading comments; while it
did in the end detect if null had not been handled correctly, that only
worked because the field `JsonAdapterFactory.recursiveCall` is static and
one test method therefore affected the state of the other test method.
If the test methods were run separately in different test runs, they would
not have detected if null was handled correctly, because the factory would
not have returned null.
* Extend JsonAdapter nullSafe test
* Extend test
* 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.
