Charset of parameters is incompatible with MySQL in `PREPARE` stage

The following go code may fail on TiDB, but success in MySQL (by default):

```
db.Query("SELECT WEIGHT_STRING(? collate utf8mb4_general_ci)", fmt.Sprintf("%c", rune(i)))
```

It'll report the following error:

```
panic: Error 1253 (42000): COLLATION 'utf8mb4_general_ci' is not valid for CHARACTER SET 'binary'
```

A workaround is to convert the `?` to `utf8mb4` explicitly:

```
db.Query("SELECT WEIGHT_STRING(CONVERT(? USING utf8mb4) collate utf8mb4_general_ci)", fmt.Sprintf("%c", rune(i)))
```