'Sets the response\'s Etag header using ``self.compute_etag()``.
Note: no header will be set if ``compute_etag()`` returns ``None``.
This method is called automatically when the request is finished.'

def set_etag_header(self):

    etag = self.compute_etag()
    if (etag is not None):
        self.set_header('Etag', etag)
