RSS 1.0 no consistent GUID

[jangernert]

The RSS 1.0 parser uses the auto generated GUID from Entry::default(). This means parsing a feed a second time for new articles results in duplicate articles.
One could use the url to check for duplicates. But imo it would be nicer if feed_rs generated a consistent id for each item. Probably based on the url of the article.

[markpritchard]

I was wondering whether it would be a downstream user of feed-rs that would handle this logic, but since the spec doesn't define a unique ID at all in RSS 1.0 its reasonable to do something sensible in feed-rs. I'll push up a fix.

[jangernert]

Having the users of feed_rs handle the problem would also be acceptable. But in that case the ID should be None. Having a random string is quite misleading.
Anyway thank you for the fix =)