mysql field: expose API to modify field value

[walkline]

In my use case, I have a kind of MySQL proxy that can modify values for certain fields, but I couldn't find an API that allows this. So, I came up with this solution, which can be used as follows:

func (p *Proxy) HandleStmtExecute(context interface{}, query string, args []interface{}) (*mysql.Result, error) {
	var stmt *client.Stmt
        //...
	res, err := stmt.Execute(args...)
        //...
        res.Values[row][column] = mysql.NewFieldValue(mysql.FieldValueTypeUnsigned, rawGUID, []byte(fmt.Sprintf("%d", rawGUID)))
	return res, nil
}