Return the part/key so that the user can manage multipart uploads

Currently uploading a part does not return the key. The key is
important as it contains the server generated etag which may
then be used by the user to complete the multipart upload
without asking the server for a list of parts.

Note: I've been using this patch for over a year now to good effect. I rebased it on top of develop again today and re-tested my application.