Allow caching with better pubdate/last-modified values

[nickvergessen]

Steps

1. Request the feed of an instance
2. Request the feed again

Expected

1. There is a last modified header that is the pubDate of the <rss><channel> element
2. The pubDate and lastBuildDate of the <rss><channel> element should be the pubDate of the newest <rss><channel><item>

Actually

1. There is no last modified header ❌
2. The pubDate and lastBuildDate are always the current time ❌