PIG-3420: Failed to retrieve map values from data loaded by AvroStorage (yuanlid via rohini)

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1514523 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk
Merge remote branch 'origin/trunk' into trunk-gd