def to_stream(confirm_valid=false)
      return false unless !confirm_valid || self.validate.empty?
      Relationship.clear_cached_instances
      zip = write_parts(Zip::OutputStream.new(StringIO.new, true))
      stream = zip.close_buffer
      stream.rewind
      stream
    end
Serialize your workbook to a StringIO instance
@param [Boolean] confirm_valid Validate the package prior to serialization.
@return [StringIO|Boolean] False if confirm_valid and validation errors exist. rewound string IO if not.