Small performance improvements

[Paxa]

• In simple cases DataTypeValidator.validate is slower then v.is_a?(ClassName)
• %{< ... } is faster then str << ('<c r="' << ...)
• values.size.times do |index| is faster then values.each_with_index do |value, index|
• DateTimeConverter.date_to_serial ~ 1.8 times faster, because avoid creating date object and making rational calculation each time. I try date.to_time.to_i and date.strftime("%s").to_i - second is much faster. But it rounds to seconds

Overall improvement is small, but for each specific method about 50% faster when compare with benchmark-ips