kiji-scoring: SCORE-164. FreshRequestOptions.Builder.addParameters should return this instead of null

issue: https://jira.kiji.org/browse/SCORE-164
review: https://review.kiji.org/r/1684
commit: https://github.com/kijiproject/kiji-scoring/commit/6c5c03e5376431af3664353f8fc2e69514c30e62