kiji-schema: SCHEMA-602. AvroUtils: reader schema union { "type" } should be compatible with writer schema "type"

issue: https://jira.kiji.org/browse/SCHEMA-602
review: https://review.kiji.org/r/1420
commit: https://github.com/kijiproject/kiji-schema/commit/92e91bf56c46e310bd0efe4ea21943ef2c88e846