def run_xml_string(cell, str = '')
        if cell.is_text_run?
          valid = RichTextRun::INLINE_STYLES - [:value, :type]
          data = Hash[cell.instance_values.map{ |k, v| [k.to_sym, v] }]
          data = data.select { |key, value| valid.include?(key) && !value.nil? }
          RichText.new(cell.value.to_s, data).to_xml_string(str)
        elsif cell.contains_rich_text?
          cell.value.to_xml_string(str)
        else
          str << ('<t>' << cell.clean_value << '</t>')
        end
        str
      end
builds an xml text run based on this cells attributes.
@param [String] str The string instance this run will be concated to.
@return [String]