[SPARK-12560][SQL] SqlTestUtils.stripSparkFilter needs to copy utf8strings

See https://issues.apache.org/jira/browse/SPARK-12560

This isn't causing any problems currently because the tests for string predicate pushdown are currently disabled.  I ran into this while trying to turn them back on with a different version of parquet.  Figure it was good to fix now in any case.

Author: Imran Rashid <irashid@cloudera.com>

Closes #10510 from squito/SPARK-12560.