kiji-express: EXP-282. Cannot pack to specific objects with snake_case fields

review: https://review.kiji.org/r/1333
issue: https://jira.kiji.org/EXP-282
commit: https://github.com/kijiproject/kiji-express/commit/29368d1088f01fbf3010eb97f62f8c7e3140d50e