Fix class-casting error in mixed clusters for 2.2->3.0 upgrades

Three upgrade tests:

• upgrade_tests/cql_tests.py:TestCQL.cas_and_list_index_test
• upgrade_tests/cql_tests.py:TestCQL.collection_and_regular_test
• upgrade_tests/cql_tests.py:TestCQL.composite_index_collections_test

fail on the upgrade path from 2.2 to 3.0. The failures can be found on CassCI here:

cas_and_list_index_test
collection_and_regular_test
composite_index_collections_test

You can run these tests with the following command:

SKIP=false CASSANDRA_VERSION=binary:2.2.0 UPGRADE_TO=git:cassandra-3.0 nosetests 2>&1 upgrade_tests/cql_tests.py:TestCQL.cas_and_list_index_test upgrade_tests/cql_tests.py:TestCQL.collection_and_regular_test upgrade_tests/cql_tests.py:TestCQL.composite_index_collections_test

Once this dtest PR is merged, these tests should also run with this upgrade path on normal 3.0 jobs.

EDIT: the following test seems to fail with the same error:

http://cassci.datastax.com/view/Upgrades/job/storage_engine_upgrade_dtest-22_tarball-30_HEAD/41/testReport/upgrade_tests.cql_tests/TestCQL/null_support_test/