code: def write io_path_or_writer
        if io_path_or_writer.is_a? Writer
          io_path_or_writer.write self
        else
          writer(io_path_or_writer).write(self)
        end
      end
docs: Write this Workbook to a File, IO Stream or Writer Object. The latter will
   make more sense once there are more than just an Excel-Writer available.