[Avro] Fix MapWriteContext not correctly resolving union values

[baharclerode]

We recently discovered a bug with the Avro serializer where using a map whose values are a union of multiple record schemas fails because the MapWriteContext does not pass each item to createChildObjectContext(), so Jackson can't resolve which schema in the union should be used.

I've added a test that demonstrates the issue and the associated fix in MapWriteContext.

It looks like this issue was fixed upstream as a part of the refactoring in 8b9171c for 3.0, but we'd like to utilize this in 2.9.

[cowtowncoder]

Thank you for the fix! Sounds good, and based on earlier work I assume this to be valid.
But I will try to go over this just to make sure I understand it. Should make it in 2.9.6.