Fix autotest failures in QAbstractNetworkCache

Fix autotest failures in QAbstractNetworkCache

The cache expiration calculation was previously removed to avoid
calculating it every time an object is fetched from the cache.
This is because the expiration date is calculated from max-age or
expires headers when the object is added to the cache.

However this broke the case where the server only provides a
last-modified header. In this case, the expiration date needs
to be checked at fetch time in order to add the warning if the
object is >24h old.

Change-Id: Iad381da0f91c27ac603abbaa94f268700ad2297f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>