Before, the KotlinJsonAdapter threw "java.lang.IndexOutOfBoundsException: Index: 0, Size: 0" on a duplicate key when the key matched a property that was not a constructor parameter.
* Fall back to reflection when generated adapter is not found.
This is handy for development builds where kapt is disabled and models still have @JsonClass(generatedAdapter = true).
* Add test for Kotlin reflection fallback behavior.
