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
da360462a5