Fix test bug in TestStemmerOverrideFilter.

This test cannot pass with a random map if ignoring case since the map might
contain the same key twice (once lowercased and once uppercased).


git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1640159 13f79535-47bb-0310-9956-ffa450edef68