No longer erroneously calculate upload_date within some extractors

`AftonbladetIE`, `BlinkxIE` and `MailRuIE` previously used `datetime.datetime.fromtimestamp` to calculate the `upload_date`. This is not only completely unnecessary, as `YoutubeDL` already does the conversion if no `upload_date` is given, but also introduced as subtle locale-dependent bug: `fromtimestamp` converts to the local timezone *of the user*, the correct call is `utcfromtimestamp`, which is correctly handled.

With these changes, `test_download` successfully works for all of these extractors, even here in Australia, where it previously reported a test failure because of this bug.