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 not sub:
                    return make_error(message='Unable to find subscription for given ARN', code=400)
                attr_name = req_data['AttributeName'][0]
                attr_value = req_data['AttributeValue'][0]
                sub[attr_name] = attr_value
                return make_response(req_action)
            elif req_action == 'GetSubscriptionAttributes':
                sub = get_subscription_by_arn(req_data['SubscriptionArn'][0])
                if not sub:
                    return make_error(message='Unable to find subscription for given ARN', code=400)
                content = '<Attributes>'
                for key, value in sub.items():
                    content += '<entry><key>%s</key><value>%s</value></entry>\n' % (key, value)
                content += '</Attributes>'
                return make_response(req_action, content=content)
            elif req_action == 'Subscribe':
                if 'Endpoint' not in req_data:
                    return make_error(message='Endpoint not specified in subscription', code=400)
            elif req_action == 'Unsubscribe':
                if 'SubscriptionArn' not in req_data:
                    return make_error(message='SubscriptionArn not specified in unsubscribe request', code=400)
                do_unsubscribe(req_data.get('SubscriptionArn')[0])
            elif req_action == 'DeleteTopic':
                do_delete_topic(topic_arn)
            elif req_action == 'Publish':
                publish_message(topic_arn, req_data)
                
                return make_response(req_action)
        return True
    def return_response(self, method, path, data, headers, response):