def to_xml_string(str = '')
      str << '<?xml version="1.0" encoding="UTF-8"?>'
      str << ('<Relationships xmlns="' << RELS_R << '">')
      each{ |rel| rel.to_xml_string(str) }
      str << '</Relationships>'
    end
def to_xml_string(str = '')
      str << '<?xml version="1.0" encoding="UTF-8"?>'
      str << ('<Relationships xmlns="' << RELS_R << '">')
      each{ |rel| rel.to_xml_string(str) }
      str << '</Relationships>'
    end
serialize relationships
 @param [String] str
 @return [String]