Test cases and fixes for Atom and RSS content decoding.

Test cases and fixes for Atom and RSS content decoding.

Fix extraction of Atom <content type="text"> and <content type="html">; we were failing to escape plaintext source data to HTML, and doing an extraneous double-deescape on HTML source resulting in breakage of notices containing text that looks like HTML. Only <content type="xhtml"> was working correctly previously.
Fixes for RSS2 content processing: we were failing to load <content:encoded> at all due to using wrong element name, and were applying an extraneous de-escape for <description> rather than the escaping that is required to turn plaintext into HTML. (Per spec, <description> must be plaintext.)