ParseDSN doesn't support usernames containing colons

### Issue description
ParseDSN unspoport ":" in username

### Example code
```go

func main() {
config := &mysql.Config{
User: "user:db",
Passwd: "test",
Net: "tcp",
Addr: "127.0.0.1",
DBName: "test",
}

s := config.FormatDSN()
fmt.Printf("dsn: %s\n", s)

config2, err := mysql.ParseDSN(s)
if err != nil {
log.Fatal(err)
}
fmt.Printf("user: %s, pass: %s\n", config2.User, config2.Passwd)

}

```