Add setContent() for Atom feed items

In the process of rolling our stuff over to FeedWriter, I realized we needed to be able to include the full content of entries. So:

- Make it easy to get a `<content>` element on Atom entries
- Spell "enclosure" correctly - "setEncloser" -> "setEnclosure"
- Fix Atom test file on case-sensitive filesystems
- Tack a `date_default_timezone_set()` on tests, to avoid warnings

I contemplated a pretty major overhaul of the library while looking at this, but I think anything I'd do would amount to a full rewrite, and I don't think I'm quite ready to spend that much time reading RFCs and fighting with validators at the moment. :)