TIKA-1133: Ability to Allow Empty and Duplicate Tika Values for XML Elements
   - Added constructors in ElementMetadataHandler to specify allowing duplicates and empty values
   - Added a unit test and test data which confirms the default and override behaviors


git-svn-id: https://svn.apache.org/repos/asf/tika/trunk@1491680 13f79535-47bb-0310-9956-ffa450edef68