Exported cookies have always a date set, even when they shouldn't

[Alberto Mardegan]

In qt/quick/api/oxideqquickwebcontext.cc, the CookiesRetrieved method prepares the cookies to be returned to the client. The "expirationdate" field is always set, even when the date is invalid (meaning that the cookie does not expire); this should be changed so that the field is not set if there is no expiration date.

This is not a critical bug, as it's fairly easy to workaround it in the client, but it would be nice to have it fixed, sometime.