* move filename/filetype validation to earlier in object creation cycle, from BUILD to _build_filetype, as empty args to object causing undef value errors in _build_filetype before finally dying in BUILD block * replaced try/catch with Test::Exception throws_ok

* replace global xlsx_data var with class attribute to prevent data persistence when object created again in same scope * new attribute to prevent close() being called more than once on xlsx_data string, though probably no harm if it does * new test file