Add a test CSV file, based on the Excel one, and a unit test shows it gets detected correctly. Also mark CSV as explicitly being a child of text/plain, rather than the previous implicit definition, to avoid confusion. TIKA-1335

git-svn-id: https://svn.apache.org/repos/asf/tika/trunk@1602352 13f79535-47bb-0310-9956-ffa450edef68