'Start pulling data from the backends so that we can learn things like
the real Content-Type that might only be in the multipart/byteranges
response body. Update our response accordingly.
Also, this is the first point at which we can learn the MIME
boundary that our response has in the headers. We grab that so we
can also use it in the body.
:returns: None
:raises HTTPException: on error'

def kickoff(self, req, resp):

    self.mime_boundary = resp.boundary
    self.stashed_iter = reiterate(self._real_iter(req, resp.headers))
    if (self.learned_content_type is not None):
        resp.content_type = self.learned_content_type
    resp.content_length = self.obj_length
