PIG-3355: ColumnMapKeyPrune bug with distinct operator (jeremykarn via aniket486)

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1494026 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk
Merge remote branch 'origin/trunk' into trunk-gd