svn merge -c 1509075 from trunk for HDFS-4513. Clarify in the WebHDFS REST API that all JSON respsonses may contain additional properties.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1509077 13f79535-47bb-0310-9956-ffa450edef68