def to_xml_string(str = '')
      str << ('<styleSheet xmlns="' << XML_NS << '">')
      [:numFmts, :fonts, :fills, :borders, :cellStyleXfs, :cellXfs, :cellStyles, :dxfs, :tableStyles].each do |key|
        self.instance_values[key.to_s].to_xml_string(str) unless self.instance_values[key.to_s].nil?
      end
      str << '</styleSheet>'
    end
def to_xml_string(str = '')
      str << ('<styleSheet xmlns="' << XML_NS << '">')
      [:numFmts, :fonts, :fills, :borders, :cellStyleXfs, :cellXfs, :cellStyles, :dxfs, :tableStyles].each do |key|
        self.instance_values[key.to_s].to_xml_string(str) unless self.instance_values[key.to_s].nil?
      end
      str << '</styleSheet>'
    end
Serializes the object
 @param [String] str
 @return [String]