XML elements in data files are written in canonical order. For example,
<title> element of the feed was at the back before, but now is in front.
Fixed a bug that autodiscovery raises
AttributeError when the given HTML contains <link> to
both application/atom+xml and application/rss+xml.
[issue #40]
Fill <title> to <description> if there’s no <title>
(rss2).
Fill <id> to the feed URL if there’s no <id>
(atom).