func (m caseInsensitiveMap) Lookup(key string) (string, bool) {
	if len(m) == 0 {
		return "", false
	}
	for _, k := range []string{
		key,
		strings.ToLower(key),
		http.CanonicalHeaderKey(key),
	} {
		v, ok := m[k]
		if ok {
			return v, ok
		}
	}
	return "", false
}