Fix for MONDRIAN-1123.  Added test suggested by Julian to Olap4jTest which
replicated the error in the 3.5 branch.  The real problem seems to be the
schema reader including these members, but I wanted my change to be very
low risk.  The change simply avoids the ClassCastException by testing the
member via instanceof before the cast.  It's unclear if any other bugs
exist due to the underlying issue with the SchemaReader, but apparently
this bug is fixed in the 4.0 branch anyways, so it might be irrelevant.