kiji-scoring: SCORE-192. Fix String/UTF8 error in test

issue: https://jira.kiji.org/browse/SCORE-192
review: https://review.kiji.org/r/2173
commit: https://github.com/kijiproject/kiji-scoring/commit/853a9c9dd10944e5d5a5a1940cc50469837b18a9