JsonObjectConverter: Ignore all final backing-fields

[stangls]

Problem:
Assume a class, that has two constructors (see test TwoConstructorsWithFinalField):

• one primary constructor with value-parameters. (JVM final fields)
• a secondary constructor.

When instantiated using Klaxon from JSON, the final fields of the primary constructor can not be filled , which may result in a non-neccessary Exception.
java.lang.IllegalArgumentException: Can not set final {{typeX}} field {{...}} to {{typeX or typeY}}

Solution:
Ignore final fields after instantiating the class in Klaxon object converter.

Reasoning:
Final fields can not be set to a new value.
All final fields can be ignored by Klaxon after instantiation, because (in JVM) they must have already been initialized during construction of the object.