Get the message associated to the status code, return null in unknown

Property definitions

nitcorn $ HttpStatusCodes :: []
	# Get the message associated to the status `code`, return `null` in unknown
	fun [](code: Int): nullable String
	do
		if codes.keys.has(code) then
			return codes[code]
		else return null
	end
lib/nitcorn/http_response.nit:121,2--127,4