For such types, the generated proxy class inherited from
java.util.AbstractSet<BoxedType<T>> (where BoxedType<T> is "Integer",
for example, when T is "int"), but defined an overloaded add() taking T,
instead of overriding the base class virtual add() taking BoxedType<T>,
resulting in an exception being thrown whenever add() was called during
run-time.
Extend Java unit test to bring it to parity with C# one added in the
previous commit.
See #1568.
This is just a mistake remaining from generalizing the old
string-specific typemap to any type.
Fix it now and update a unit test to test for sets of objects other than
strings.
