change mysql default charset

lualib/skynet/db/mysql.lua

@@ -674,7 +674,7 @@ function _M.connect(opts)
     local database = opts.database or ""
     local user = opts.user or ""
     local password = opts.password or ""
-    local charset = CHARSET_MAP[opts.charset or "utf8mb4"]
+    local charset = CHARSET_MAP[opts.charset or "_default"]
     local channel = 
         socketchannel.channel {
         host = opts.host,

test/testmysql.lua

@@ -128,6 +128,7 @@ skynet.start(function()
 		database="skynet",
 		user="root",
 		password="123456",
+        charset="utf8mb4",
 		max_packet_size = 1024 * 1024,
 		on_connect = on_connect
 	})