PIG-3827: Custom partitioner is not picked up with secondary sort optimization

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/branch-0.12@1580090 13f79535-47bb-0310-9956-ffa450edef68
PIG-3827: Custom partitioner is not picked up with secondary sort optimization

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/branch-0.12@1580093 13f79535-47bb-0310-9956-ffa450edef68