Fix for TIKA-1369: Resolve thread safety issue in ImageMetadataExtractor. Contributed by Vilmos Papp <papp.gyorgy.vilmos@gmail.com>. This closes #15.

git-svn-id: https://svn.apache.org/repos/asf/tika/trunk@1629347 13f79535-47bb-0310-9956-ffa450edef68