Comment about bad Expires header doesn't match semantics

[glasser]

The code reads:

	if respHeaders.Get("Expires") != "" {
		expiresHeader, err = http.ParseTime(respHeaders.Get("Expires"))
		if err != nil {
			// sometimes servers will return `Expires: 0` or `Expires: -1` to
			// indicate expired content
			expiresHeader = time.Time{}
		}
		expiresHeader = expiresHeader.UTC()
	}

The comment says that Expires:0 or Expires:-1 means expired content. But the code doesn't treat it any differently than no Expires header.