This is establishing the API for a future optimization. We want to be able to offer true socket-level caching, but can't do that with httplib2 right now. For now, we will just fake the optimization by returning an iterator over the image body, which happens to already be fully loaded into a string. Change-Id: I2d36e3cdd45b26d7c7c27ba050bf6a4b5765df6c