def to_xml_string(str = '')
      str << ('<conditionalFormatting sqref="' << sqref << '">')
      str << rules.collect{ |rule| rule.to_xml_string }.join(' ')
      str << '</conditionalFormatting>'
    end
def to_xml_string(str = '')
      str << ('<conditionalFormatting sqref="' << sqref << '">')
      str << rules.collect{ |rule| rule.to_xml_string }.join(' ')
      str << '</conditionalFormatting>'
    end
Serializes the conditional formatting element
 @example Conditional Formatting XML looks like:
    <conditionalFormatting sqref="E3:E9">
        <cfRule type="cellIs" dxfId="0" priority="1" operator="greaterThan">
             <formula>0.5</formula>
        </cfRule>
    </conditionalFormatting>
 @param [String] str
 @return [String]