def formula_serialization(cell, str='')
        str << ('t="str"><f>' << cell.clean_value.to_s.sub('=', '') << '</f>')
        str << ('<v>' << cell.formula_value.to_s << '</v>') unless cell.formula_value.nil?
      end
Serializes cells that are type formula
@param [Cell] cell The cell that is being serialized
@param [String] str The string the serialized content will be appended to.
@return [String]