`charset` option does not work.

[furusax0621]

Since v1.9.0, when initializing sql.DB with the charset option set in config.Params as shown in the following code, an error occurs during query execution:

config := mysql.NewConfig()
config.Net = "tcp"
config.Addr = "127.0.0.1:3306"
// etc...
config.Params = map[string]string{
    "charset": "utf8mb4",
}

conn, err := mysql.NewConnector(config)
if err != nil {
    log.Fatal(err)
}
db := sql.OpenDB(conn)

if _, err := db.Exec("SELECT 1"); err != nil {
    log.Fatal(err)
}
// return Error 1193 (HY000): Unknown system variable 'charset'

However, it works without any issues when initialized using config.FormatDSN():

config := mysql.NewConfig()
config.Net = "tcp"
config.Addr = "127.0.0.1:3306"
// etc...
config.Params = map[string]string{
    "charset": "utf8mb4",
}
db, err := sql.Open("mysql", config.FormatDSN())
if err != nil {
    log.Fatal(err)
}

if _, err := db.Exec("SELECT 1"); err != nil {
    log.Fatal(err)
}

I believe this is due to the change in the timing of interpreting the charset option in #1604 .
In the current implementation, it is interpreted within the ParseDSN method and stored in the config.charsets field.

mysql/dsn.go

Lines 525 to 527 in 58941dd

	// charset
	case "charset":
	cfg.charsets = strings.Split(value, ",")

However, in the way I wrote, the connection is created without going through the ParseDSN method, so charset is sent as an invalid option.

The charset option should be interpreted regardless of the connection establishment procedure.

[methane]

Duplicate of #1664 and #1678.

See #1679 as fix PR.

And using mysql.ParseDSN("/?charset=utf8mb4") instead of mysql.NewConfig() is workaround for now.