kiji-scoring: SCORE-172. Improve JpmmlScoreFunction.

Handles more corner cases in PMML models.  Unit tests upgraded
accordingly.

review: https://review.kiji.org/r/1742
issue: https://jira.kiji.org/SCORE-172
commit: https://github.com/kijiproject/kiji-scoring/commit/62c65f1c314f1da716741a8d2597e1c0acc259cd