Fix for polymorphic custom map key serializer

[mjr6140]

When using ObjectMapper.DefaultTyping.NON_FINAL with a non-final map key
and custom map key serializer the serializer is being ignored and
instead the default serializer is always called.

See test case for a minimal example of the issue. It looks like this issue appeared
starting with 2.5.0 but worked correctly in previous versions.

[cowtowncoder]

Sounds plausible. I may need to backport this into 2.5 branch if so.

[cowtowncoder]

I hope you don't mind if I actually include test first, for 2.5; may do manual merge here.

[cowtowncoder]

Ok: I made one minor change to serializer accessor, since I think it should not try wrapping with type serializer, but otherwise fixed as suggested.
But unlike my earlier plan, I did not backport fix in 2.5.x yet. My concern is that with JAX-RS it could lead to issues (there are a few damned-if-you-damned-if-you-dont dilemmas) so I'd rather go with 2.6 route.

I also really need to figure out the distinction between "root serialization type" and "root base type for polymorphic serialization", but one step at a time.

Thank you for reporting this and pointing out the fix!

[mjr6140]

Thanks for taking a look at this! I just re-tested and this is working as expected now in master.