kiji-mapreduce-lib: KIJIMRLIB-26. Fix IdentityProducer

issue: https://jira.kiji.org/browse/KIJIMRLIB-26
review: https://review.kiji.org/r/1730
commit: https://github.com/kijiproject/kiji-mapreduce-lib/commit/e4e58c4ae0ae89f66c0f189636b660a25e3b7e9d