def to_xml_string(str = '')
      str << '<?xml version="1.0" encoding="UTF-8"?>'
      str << ('<cp:coreProperties xmlns:cp="' << CORE_NS << '" xmlns:dc="' << CORE_NS_DC << '" ')
      str << ('xmlns:dcmitype="' << CORE_NS_DCMIT << '" xmlns:dcterms="' << CORE_NS_DCT << '" ')
      str << ('xmlns:xsi="' << CORE_NS_XSI << '">')
      str << ('<dc:creator>' << self.creator << '</dc:creator>')
      str << ('<dcterms:created xsi:type="dcterms:W3CDTF">' << (created || Time.now).strftime('%Y-%m-%dT%H:%M:%S') << 'Z</dcterms:created>')
      str << '<cp:revision>0</cp:revision>'
      str << '</cp:coreProperties>'
    end
serializes the core.xml document
@return [String]