Make date parsing work in download()

download() was setting feed.linkdate to localtime regardless of whether entry.published_parsed or entry.update_parsed existed.
This means that if you use the placeholder {date} in the filename (or anywhere), it will not reflect the original published date, but rather the date you download it.
sync() declares feed.linkdate properly, the problem only occurs when download() is used. I basically copied the appropriate snippet from sync() to download()