use async dns resolve in httpc, and dns resolve cache by ttl. see about issue #253

lualib/http/httpc.lua

@@ -1,6 +1,7 @@
 local socket = require "http.sockethelper"
 local url = require "http.url"
 local internal = require "http.internal"
+local dns = require "dns"
 local string = string
 local table = table
 
@@ -78,6 +79,13 @@ local function request(fd, method, host, url, recvheader, header, content)
 	return code, body
 end
 
+local async_dns
+
+function httpc.dns(server,port)
+	async_dns = true
+	dns.server(server,port)
+end
+
 function httpc.request(method, host, url, recvheader, header, content)
 	local hostname, port = host:match"([^:]+):?(%d*)$"
 	if port == "" then
@@ -85,6 +93,9 @@ function httpc.request(method, host, url, recvheader, header, content)
 	else
 		port = tonumber(port)
 	end
+	if async_dns and not hostname:match(".*%d+$") then
+		hostname = dns.resolve(hostname)
+	end
 	local fd = socket.connect(hostname, port)
 	local ok , statuscode, body = pcall(request, fd,method, host, url, recvheader, header, content)
 	socket.close(fd)