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. 
queue_url, queue_attributes = self._create_test_queue()
        self._create_test_notification_bucket(queue_attributes)
        
        file_size = 75000000
        large_file = self.generate_large_file(file_size)
        download_file = new_tmp_file()
        try:
            self.s3_client.upload_file(Bucket=TEST_BUCKET_WITH_NOTIFICATION,
                                       Key=large_file.name, Filename=large_file.name)
            self.assertEqual(self._get_test_queue_message_count(queue_url), '1')
            
            messages = self.sqs_client.receive_message(QueueUrl=queue_url, AttributeNames=['All'])
            message = json.loads(messages['Messages'][0]['Body'])
            self.assertEqual(message['Records'][0]['eventName'], 'ObjectCreated:CompleteMultipartUpload')
            
            self.s3_client.download_file(Bucket=TEST_BUCKET_WITH_NOTIFICATION,
                                         Key=large_file.name, Filename=download_file)
            self.assertEqual(os.path.getsize(download_file), file_size)
            
            self.sqs_client.delete_queue(QueueUrl=queue_url)
            self._delete_bucket(TEST_BUCKET_WITH_NOTIFICATION, large_file.name)
        finally:
            
            large_file.close()
            rm_rf(large_file.name)
            rm_rf(download_file)
    def test_s3_multipart_upload_with_small_single_part(self):