def to_xml_string(str = '')
      update_properties
      str << "<sheetPr #{serialized_attributes}>"
      tab_color.to_xml_string(str, 'tabColor') if tab_color
      outline_pr.to_xml_string(str) if @outline_pr
      page_setup_pr.to_xml_string(str)
      str << "</sheetPr>"
    end
def to_xml_string(str = '')
      update_properties
      str << "<sheetPr #{serialized_attributes}>"
      tab_color.to_xml_string(str, 'tabColor') if tab_color
      outline_pr.to_xml_string(str) if @outline_pr
      page_setup_pr.to_xml_string(str)
      str << "</sheetPr>"
    end
Serialize the object
 @param [String] str serialized output will be appended to this object if provided.
 @return [String]