Get a list of headers.

def get_response(self, environ=None):
        """Get a list of headers."""
        response = super(SameContentException, self).get_response(
            environ=environ
        )
        if self.etag is not None:
            response.set_etag(self.etag)
        if self.last_modified is not None:
            response.headers['Last-Modified'] = http_date(self.last_modified)
        return response