kiji-express: EXP-291. Fix AvroTupleConversions serialization bug

issue: https://jira.kiji.org/browse/EXP-291
review: https://review.kiji.org/r/1348
commit: https://github.com/kijiproject/kiji-express/commit/c487005d55836e343ad2962b6706b149f3ff1bd7