c.Assert(err, nil)
	request.Header.Set("If-Unmodified-Since", t.Add(-10*time.Minute).UTC().Format(http.TimeFormat))
	response, err = s.client.Do(request)
	c.Assert(err, nil)
	c.Assert(response.StatusCode, http.StatusPreconditionFailed)

	// make HTTP request to obtain object info.
	// But this time set a date with unrecognized format to the "If-Modified-Since" header