kiji-express: EXP-286. rename ColumnRequestInput#schema -> schemaSpec.

issue: https://jira.kiji.org/browse/EXP-286
review: https://review.kiji.org/r/1338
commit: https://github.com/kijiproject/kiji-express/commit/206fd9d7fc1353e74228ee4be5728430fd6bb7dc