fix Kotlin master (3.0) test failures -- due to MapperFeature.ALLOW_FINAL_FIELDS_AS_MUTATORS default change;

[JooHyukKim]

addresses part of #807

[cowtowncoder]

@JooHyukKim There may have been a misunderstanding here -- I did not mean Kotlin module should use different setting, but that test classes would be changed to use old setting.
Default behavior of the module should not differ from jackson-databind in 3.0.

Would it be possible to just change ObjectMapper configuration just for tests that are failing?
(ideally tests would not rely on this, but on short term they may).

[JooHyukKim]

Ah, okay. will do 👍🏼