looks like setting ExposeHeaders has no effect on requests to s3 (headers such as `x-amz-version-id` don't reach the clients) for browser-based uploads. 

http://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectPOST.html
import re
import logging
import json
import uuid
import base64
import codecs
import xmltodict
import collections
import botocore.config
import six
from six import iteritems