Don't always mark feed items as unread if the pubDate of the item is unknown

Fixes the most recent (as of the moment of writing) item of http://what-if.xkcd.com/feed.atom always being marked as unread due to it having an invalid pubDate.

If an item has an invalid pubDate simplepie will return ```false``` as pubDate, when this is stored in the db it's casted to ```0``` and ```0 !== false```

@Raydiation @zimba12