SI-6364 SetWrapper does not preserve performance / behavior

O(n) performance of wrapped set contains was the problem.

Added overrides for contains and isEmpty to SetWrapper.  Note that sets are
invariant in Scala, while the Java signature is for any Object, so we trap
a ClassCastException if one occurs.  (Is this everything that could possibly
go wrong?  I think so, but am not as confident as I would like.)
Merge pull request #3302 from Ichoran/issue/6364

Resolves SI-6364, O(n) performance of wrapped set contains.