# Type of request: This is a bug report

# Detailed description
S3 list buckets returns a different format for CreationDate than the one expected by the Java SDK and parsing fails. Using Java SDK 2.13.39. Also verified w/ .40, .38, .30 and .20. Will check previous Docker releases for localstack.

``Unable to unmarshall response (Text '2020-06-17T15:26:46.032838+00:00' could not be parsed at index 19). Response Code: 200, Response Text: 
software.amazon.awssdk.core.exception.SdkClientException: Unable to unmarshall response (Text '2020-06-17T15:26:46.032838+00:00' could not be parsed at index 19). Response Code: 200, Response Text: <truncated...>
``

## Expected behavior
Return CreationDate in same format as the actual S3 API call.

## Actual behavior
Returns CreationDate in slightly different format from the actual S3 API call. Fractional seconds are included.

# Steps to reproduce
Run "aws s3api list-buckets" vs "aws s3api list-buckets --endpoint-url http://localstack:4566" and note different date format for CreationDate.

## Command used to start LocalStack
docker-compose:
```
  localstack:
    image: localstack/localstack
    ports:
      - '4566:4566'
    environment:
      SERVICES: s3
      DEBUG: 1
```
## Client code (AWS SDK code snippet, or sequence of "awslocal" commands)
See steps to reproduce.
import re
import json
import uuid
import base64
import codecs
import random
import logging
import datetime
import xmltodict
import collections
import dateutil.parser
import six
import botocore.config
from pytz import timezone
from botocore.client import ClientError
from requests.models import Response, Request
from six.moves.urllib import parse as urlparse
from localstack import config, constants
from localstack.config import HOSTNAME, HOSTNAME_EXTERNAL
from localstack.utils.aws import aws_stack
from localstack.services.s3 import multipart_content
from localstack.utils.common import (
    short_uid, timestamp_millis, to_str, to_bytes, clone, md5, get_service_protocol, now_utc
)