使用Go和GSM或ASCII编码通过AWS SNS发送长短信

Hi I read on AWS SNS documentation that SMS messages sent using GSM-7 encoding can have 160 characters or 140 on ASCII or 70 on UC-2. The SNS will select the best encoding for the message. I'm using the following code to send SMS messages and it always selects the UC-2 encoding breaking my messages in two or more parts. There is a way to force SNS use ASCII or GSM encoding ?

svc := sns.New(newSession())

if _, err = svc.Publish(&sns.PublishInput{
    Message:     aws.String(content),
    PhoneNumber: aws.String(number),
    MessageAttributes: map[string]*sns.MessageAttributeValue{
        "DefaultSMSType": {
             DataType: aws.String("String"),
             StringValue: aws.String("Transactional")},
        },
}); err != nil {
    log.Println(err.Error())
    return
}