func getContentChecksum(h http.Header) (t ChecksumType, s string) {
	t = ChecksumNone
	alg := h.Get(xhttp.AmzChecksumAlgo)
	if alg != "" {
		t |= NewChecksumType(alg)
		if t.IsSet() {
			hdr := t.Key()
			if s = h.Get(hdr); s == "" {
				return ChecksumNone, ""
			}
		}
		return t, s
	}
	checkType := func(c ChecksumType) {
		if got := h.Get(c.Key()); got != "" {
			// If already set, invalid