idtoken caching does not handle missing age in http response header

[eytankidron]

In function calculateExpireTime (see

google-api-go-client/idtoken/cache.go

Lines 98 to 119 in 977e871

	func (c *cachingClient) calculateExpireTime(headers http.Header) time.Time {
	var maxAge int
	cc := strings.Split(headers.Get("cache-control"), ",")
	for _, v := range cc {
	if strings.Contains(v, "max-age") {
	ss := strings.Split(v, "=")
	if len(ss) < 2 {
	return c.now()
	}
	ma, err := strconv.Atoi(ss[1])
	if err != nil {
	return c.now()
	}
	maxAge = ma
	}
	}
	age, err := strconv.Atoi(headers.Get("age"))
	if err != nil {
	return c.now()
	}
	return c.now().Add(time.Duration(maxAge-age) * time.Second)
	}

) the code expects the "age" field to be present, otherwise the caching expiration is set to "now", which effectively means that the response is not cached.

According to https://httpwg.org/specs/rfc9111.html#age.calculations, a missing "age" field in the response header should be allowed and treated as if it was 0. So I think we should allow it to be missing.

[codyoss]

This issue has been addressed and released.