kiji-modeling: MODELING-16. lift and confidence don't discard existing fields on the pipe.

issue: https://jira.kiji.org/browse/MODELING-16
review: https://review.kiji.org/r/1565
commit: https://github.com/kijiproject/kiji-modeling/commit/f1eeb5d9a943a86f5d580facf2e7a9e29a3f2847