Hi!

I have noticed that today (25th of Feb) a new version of localstack docker image was pushed to the repository (82c83ff027114060938856eac5dc7be1d534afd0b7711510246de4a76dde2396) and this version fails in my integration tests with an exception:

```
2020-02-25T12:09:44:ERROR:localstack.services.generic_proxy: Error forwarding request: An error occurred (404) when calling the HeadObject operation: Not Found Traceback (most recent call last):
  File "/opt/code/localstack/localstack/services/generic_proxy.py", line 303, in forward updated_response = self.proxy.update_listener.return_response(**kwargs)
  File "/opt/code/localstack/localstack/services/s3/s3_listener.py", line 954, in return_response
send_notifications(method, bucket_name, object_path, version_id)
  File "/opt/code/localstack/localstack/services/s3/s3_listener.py", line 178, in send_notifications
send_notification_for_subscriber(notif, bucket_name, object_path,
  File "/opt/code/localstack/localstack/services/s3/s3_listener.py", line 191, in send_notification_for_subscriber
object_size = s3_client.head_object(Bucket=bucket_name, Key=key).get('ContentLength', 0)
  File "/opt/code/localstack/.venv/lib/python3.8/site-packages/botocore/client.py", line 316, in _api_call
return self._make_api_call(operation_name, kwargs)
  File "/opt/code/localstack/.venv/lib/python3.8/site-packages/botocore/client.py", line 626, in _make_api_call
  raise error_class(parsed_response, operation_name)
botocore.exceptions.ClientError: An error occurred (404) when calling the HeadObject operation: Not Found
```
The same setup in docker-compose works with version 0.10.7 and 0.10.6 without any errors or exceptions. 
}
                        result += xmltodict.unparse(dest_dict, full_document=False)
        result += '</NotificationConfiguration>'
        response._content = result
    if method == 'PUT':
        parsed = xmltodict.parse(data)
        notif_config = parsed.get('NotificationConfiguration')
        S3_NOTIFICATIONS[bucket] = []
        for dest in NOTIFICATION_DESTINATION_TYPES:
            config = notif_config.get('%sConfiguration' % (dest))
            configs = config if isinstance(config, list) else [config] if config else []
            for config in configs:
                events = config.get('Event')
                if isinstance(events, six.string_types):
                    events = [events]
                event_filter = config.get('Filter', {})
                
                s3_filter = _get_s3_filter(event_filter)
                if s3_filter and not isinstance(s3_filter.get('FilterRule', []), list):
                    s3_filter['FilterRule'] = [s3_filter['FilterRule']]
                
                notification_details = {
                    'Id': config.get('Id'),
                    'Event': events,
                    dest: config.get(dest),
                    'Filter': event_filter
                }
                S3_NOTIFICATIONS[bucket].append(clone(notification_details))
    return response
class ProxyListenerS3(ProxyListener):