added an asMap() method

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1634048 13f79535-47bb-0310-9956-ffa450edef68
Revert backport

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1634052 13f79535-47bb-0310-9956-ffa450edef68