Thanks for the report! It turns out there is a bug in the RADOS Gateway (RGW) portion of Ceph that's responsible for handling the customer-facing API. Other operations are returning the content-type of "application/xml" but the put object copy is not. I also noticed that it's not returning the ETag in the XML either. I've created a bug report and hope to hear more soon.
I'm guessing this isn't a high priority bug so I doubt it will be fixed very soon. I did some testing with s3cmd and it seemed to handle copies without issue and I know it's using boto under the hood for it's API operations. You may want to look into adjusting your script if possible.