add a callback for mysql auth

test/testmysql.lua

@@ -70,14 +70,18 @@ local function test3( db)
 end
 skynet.start(function()
 
+	local function on_connect(db)
+		db:query("set charset utf8");
+	end
 	local db=mysql.connect({
 		host="127.0.0.1",
 		port=3306,
 		database="skynet",
 		user="root",
 		password="1",
-		max_packet_size = 1024 * 1024
-	}, function(db) db:query("set charset utf8") end)
+		max_packet_size = 1024 * 1024,
+		on_connect = on_connect
+	})
 	if not db then
 		print("failed to connect")
 	end