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 ast
import json
import logging
import requests
import uuid
import xmltodict
import six
from requests.models import Response
from six.moves.urllib import parse as urlparse
from localstack.utils.aws import aws_stack
from localstack.utils.common import short_uid, to_str
from localstack.services.awslambda import lambda_api
from localstack.services.generic_proxy import ProxyListener
SNS_SUBSCRIPTIONS = {}
LOGGER = logging.getLogger(__name__)
class ProxyListenerSNS(ProxyListener):