use OpenMode.CREATE_OR_APPEND (the default) per:

- http://irclog.iq.harvard.edu/dvn/2013-03-13#i_1428
- http://svn.apache.org/viewvc/lucene/dev/branches/branch_4x/lucene/facet/src/java/org/apache/lucene/facet/doc-files/userguide.html?r1=1456119&r2=1456118&pathrev=1456119