QNetworkCookie - use RFC6265 rules for max-age

QNetworkCookie - use RFC6265 rules for max-age

If unparsable, ignore the max-age attribute but process the rest of
the cookie normally.
If max age <= 0, set expiration time to "earliest representable time"
To keep this a safe value for conversions, time_t of 0 is used.

This fixes cases 0019 and comma0005 in the test suite.
Due to this change, cookies may be sent after they should have expired
in case the max-age was malformed. Previously they would have been
discarded immediately, which is more likely to break web services.

Task-number: QTBUG-15794
Change-Id: I7882af8eb37db156785e4e358ca639e90c94f8d0
Reviewed-by: Richard J. Moore <rich@kde.org>