Write the diff (toVol from fromVol) to the stream context manager.

def send(self, diff):
        """ Write the diff (toVol from fromVol) to the stream context manager. """
        path = self._fullPath(self.extraKeys[diff])
        keyName = self._keyName(diff.toUUID, diff.fromUUID, path)
        key = self.bucket.get_key(keyName)

        if self._skipDryRun(logger)("send %s in %s", keyName, self):
            return None

        progress = _BotoProgress(diff.size) if self.showProgress is True else None
        return _Downloader(key, progress)