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. 
import io
import os
import ssl
import gzip
import json
import uuid
import unittest
import requests
from io import BytesIO
from six.moves.urllib.request import Request, urlopen
from localstack import config
from localstack.utils.aws import aws_stack
from localstack.utils.common import (
    short_uid, get_service_protocol, to_bytes, safe_requests, to_str, new_tmp_file, rm_rf)
TEST_BUCKET_NAME_WITH_POLICY = 'test-bucket-policy-1'
TEST_BUCKET_WITH_NOTIFICATION = 'test-bucket-notification-1'
TEST_QUEUE_FOR_BUCKET_WITH_NOTIFICATION = 'test_queue_for_bucket_notification_1'
TEST_BUCKET_WITH_VERSIONING = 'test-bucket-versioning-1'
class PutRequest(Request):