JsonObjectOrFalseDeserializer doesn't work with Kotlin classes

[malinajirka]

JsonObjectOrFalseDeserializer uses java reflection Class.getFields() method here which returns an empty array since Kotlin classes don't have any fields (they have members).

This issue can lead to hard to find bugs when someone creates/converts a Kotlin class. (https://github.com/wordpress-mobile/WordPress-FluxC-Android/pull/1262)

[maxme]

This is a bigger project (but related to this), we should replace Gson by a more flexible alternative.