NetworkCookie: fix date validity check

* January 1st 1970 is a perfectly valid expiration date for a cookie,
and shouldn't magically turn it into a session cookie.

src/kits/network/libnetapi/NetworkCookie.cpp

@@ -232,7 +232,7 @@ BNetworkCookie::SetExpirationDate(time_t expireDate)
 BNetworkCookie&
 BNetworkCookie::SetExpirationDate(BDateTime& expireDate)
 {
-	if (expireDate.Time_t() <= 0) {
+	if (!expireDate.IsValid()) {
 		fExpiration.SetTime_t(0);
 		fSessionCookie = true;
 		fExpirationStringValid = false;