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 
def forward_request(self, method, path, data, headers):
        
        try:
            aws_stack.check_valid_region(headers)
        except Exception as e:
            return make_error(message=str(e), code=400)
        if method == 'POST' and path == '/':
            req_data = urlparse.parse_qs(to_str(data))
            req_action = req_data['Action'][0]
            topic_arn = req_data.get('TargetArn') or req_data.get('TopicArn')
            if topic_arn:
                topic_arn = topic_arn[0]
                do_create_topic(topic_arn)
            if req_action == 'SetSubscriptionAttributes':
                sub = get_subscription_by_arn(req_data['SubscriptionArn'][0])