Default acl

The default ACL on S3 is `private` (which doesn't necessarily mean that the file are actually private, it can be overridden by a bucket policy for instance).
So, imho, tinys3 should offer the same default settings.

The second commit is a very small bugfix in `UploadRequest._calc_cache_control`, if this PR is refused the fix should still be applied to master (yet another PR for that seemed a tiny bit overkill)

Arthur