code: public Iterable<String> toCSV() {
      checkState(!isClosed, WORKBOOK_CLOSED);
      Joiner joiner = Joiner.on(",").useForNull("");
      Iterable<String> CSVIterable =
          Iterables.transform(sheet, item -> joiner.join(rowToList(item, true)));
      return hasHeader ? Iterables.skip(CSVIterable, 1) : CSVIterable;
    }
docs: Converts the spreadsheet to CSV by a String Iterable.
  
  @return String Iterable