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 setUp(self):
        self.subscriber = {
            'Protocol': 'sqs',
            'RawMessageDelivery': 'false',
            'TopicArn': 'arn',
        }
    def test_unsubscribe_without_arn_should_error(self):
        sns = sns_listener.ProxyListenerSNS()
        error = sns.forward_request('POST', '/', 'Action=Unsubscribe', '')
        self.assertTrue(error is not None)
        self.assertEqual(error.status_code, 400)
    def test_unsubscribe_should_remove_listener(self):
        sub_arn = 'arn:aws:sns:us-east-1:123456789012:test-topic:45e61c7f-dca5-4fcd-be2b-4e1b0d6eef72'
        topic_arn = 'arn:aws:sns:us-east-1:123456789012:test-topic'
        self.assertFalse(sns_listener.get_topic_by_arn(topic_arn))
        sns_listener.do_create_topic(topic_arn)
        self.assertTrue(sns_listener.get_topic_by_arn(topic_arn) is not None)
        sns_listener.do_subscribe(
            topic_arn,
            'http://localhost:1234/listen',
            'http',
            sub_arn
        )
        self.assertTrue(sns_listener.get_subscription_by_arn(sub_arn))
        sns_listener.do_unsubscribe(sub_arn)
        self.assertFalse(sns_listener.get_subscription_by_arn(sub_arn))
    def test_create_sns_message_body_raw_message_delivery(self):