Backport HDFS-4983 to branch-2.3. Numeric usernames do not work with WebHDFS FS. Contributed by Yongjun Zhang.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.3@1549746 13f79535-47bb-0310-9956-ffa450edef68