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 
import json
import unittest
from localstack.utils.aws import aws_stack
from localstack.utils.common import to_str
TEST_TOPIC_NAME = 'TestTopic_snsTest'
TEST_QUEUE_NAME = 'TestQueue_snsTest'
class SNSTest(unittest.TestCase):
    def test_publish_unicode_chars(self):
        sqs_client = aws_stack.connect_to_service('sqs')
        sns_client = aws_stack.connect_to_service('sns')
        
        topic = sns_client.create_topic(Name=TEST_TOPIC_NAME)
        queue = sqs_client.create_queue(QueueName=TEST_QUEUE_NAME)
        topic_arn = topic['TopicArn']
        queue_url = queue['QueueUrl']
        queue_arn = aws_stack.sqs_queue_arn(TEST_QUEUE_NAME)
        sns_client.subscribe(TopicArn=topic_arn, Protocol='sqs', Endpoint=queue_arn)
        
        message = u'ö§a1"_!?,. £$-'
        sns_client.publish(TopicArn=topic_arn, Message=message)
        msgs = sqs_client.receive_message(QueueUrl=queue_url)
        msg_received = msgs['Messages'][0]
        msg_received = json.loads(to_str(msg_received['Body']))
        msg_received = msg_received['Message']
        self.assertEqual(message, msg_received)