Hello,
I am working with a SNS to SQS project, in it we want to use the feature `RawMessageBody` when subscribing.
By looking into the code I found that the attributes are not persisted, for example `RawMessageBody` is always set to 'false'. Elsewhere in the code the parameters seems to be correctly used and send transform the message accordingly.

I created a PR fixing this : https://github.com/localstack/localstack/pull/1234
Thank you 
if topic_arn not in SNS_SUBSCRIPTIONS:
        SNS_SUBSCRIPTIONS[topic_arn] = []
def do_delete_topic(topic_arn):
    if topic_arn in SNS_SUBSCRIPTIONS:
        del SNS_SUBSCRIPTIONS[topic_arn]
def do_subscribe(topic_arn, endpoint, protocol, subscription_arn):
    subscription = {
        
        'TopicArn': topic_arn,
        'Endpoint': endpoint,
        'Protocol': protocol,
        'SubscriptionArn': subscription_arn,
        'RawMessageDelivery': 'false'
    }
    SNS_SUBSCRIPTIONS[topic_arn].append(subscription)
def do_unsubscribe(subscription_arn):
    for topic_arn in SNS_SUBSCRIPTIONS:
        SNS_SUBSCRIPTIONS[topic_arn] = [
            sub for sub in SNS_SUBSCRIPTIONS[topic_arn]
            if sub['SubscriptionArn'] != subscription_arn
        ]
def get_topic_by_arn(topic_arn):
    if topic_arn in SNS_SUBSCRIPTIONS:
        return SNS_SUBSCRIPTIONS[topic_arn]
    else:
        return None
def get_subscription_by_arn(sub_arn):