'this backend should not even notice if the file is missing.'

def test_missing_file(self):

    request = Mock()
    request.META = {}
    os.remove(self.filer_file.file.path)
    response = self.server.serve(request, self.filer_file.file)
    headers = dict(response.items())
    self.assertTrue(response.has_header('X-Sendfile'))
    self.assertEqual(headers['X-Sendfile'], self.filer_file.file.path)
    self.assertTrue(self.filer_file.file.closed)
