[Fixes #615] Minimal fix for our use case, with test:
Mapper.isCompatibleForOperator should return true if the value being
queried is an instance of the mapped type of the field.  Previous
logic was swapped.
[#615] Mapper.isCompatibleForOperator: Fix for $size of ArrayList field
[#615] Mapper.isCompatibleForOperator: Fix for subclasses of Key
(Seems unlikely in practice; fixing for completeness.)
Fix Intellij "suspicious call" warnings in Mapper.isCompatibleForOperator