SetExternalizer Cannot Properly Handle TreeSets Constructed With Comparator

TreeSets can be used to hold objects that don't implement Comparable, so long as you provide a Comparator during construction. The current implementation of SetExternalizer doesn't support this pattern... trying to deserialize a TreeSet that holds objects that don't implement Comparable results in a ClassCastException. This seriously reduces the flexibility of TreeSet.

JBoss Cache doesn't have this limitation, presumably because it delegates to native Java serialization.