Fix #807 by enabling MapperFeature.ALLOW_FINAL_FIELDS_AS_MUTATORS on selected tests

[cowtowncoder]

No description provided.

[cowtowncoder]

@k163377 @JooHyukKim here are changes to work around the issues. But tests suggest that there are many cases where forcing setting of final Fields is expected for Kotlin usage. Ideally this should not be required as JVM may well stop allowing this to be done; plus, usually value should be passed through constructor anyway.
Unfortunately I wasn't able to really properly fix any of failing tests in that regard.

[JooHyukKim]

Unfortunately I wasn't able to really properly fix any of failing tests in that regard.

Yeah the handling of final fields seems to be required of Kotlin module at the moment. I also failed to come up with module-level solution that we can integrate into pre-Jackson 3.

This issue may be a blocker problem- Jackson 3 🤔

[cowtowncoder]

@JooHyukKim it is a concern yes. But I think we are ok for now -- change in defaults is not causing an issue as much as exposing it, I think (one really should not modify final field values).