Entry content isn't displayed for Atom feeds using <content> and RSS feeds using content:encoded

Steps to reproduce

• Use the RSS macro with content="true" on an Atom feed whose entries only have a <content> element (no <summary>), which is the case of many Atom feeds, or on an RSS 2.0 feed whose items only have a content:encoded element (no <description>).

Expected

The content of each feed entry is displayed below its title.

Actual

Only the entry titles are displayed: the macro only reads the entry description (Atom <summary> / RSS <description>) and ignores the entry contents.

In addition, an entry with an empty description is displayed with an empty rssitemdescription block.