Converters should use isAssignableFrom, not equals

Many xstream converters use class equality when determining if they can convert an object of a given type. This is undesirable, as it prevents users from easily serializing classes that extend these objects. Under the current implementation, users are forced to implement a custom converter in order to support this scenario. Instead, Class.isAssignableFrom should be use when checking "canConvert", so that the converters will properly convert subclasses as well. If a user still needs custom functionality they can then fall-back to creating their own converter.

So far I have found the following converters to be affected:
CollectionConverter
MapConverter
TreeMapConverter
TreeSetConverter