def to_xml_string(str='')
      return if empty?
      str << ('<colBreaks count="' << size.to_s << '" manualBreakCount="' << size.to_s << '">')
      each { |brk| brk.to_xml_string(str) }
      str << '</colBreaks>'
    end
Serialize the collection to xml
@param [String] str The string to append this lists xml to.
<colBreaks count="1" manualBreakCount="1">
<brk id="3" max="1048575" man="1"/>
</colBreaks>