perhaps simply returning the method if _headers_buffer isn't defined is better since there is probably nothing to flush.

    def end_headers(self):
        if not hasattr(self, '_headers_buffer'):
            return 1 
        """Send the blank line ending the MIME headers."""
        if self.request_version != 'HTTP/0.9':
            self._headers_buffer.append(b"\r\n")
            self.flush_headers()