class Axlsx::CellSerializer

def run_xml_string(cell, str = '')
  if cell.is_text_run?
    valid = RichTextRun::INLINE_STYLES - [:value, :type]
    data = Hash[Axlsx.instance_values_for(cell).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