Use 'saveHTML' when generating HTML from a DOMDocument.

Use 'saveHTML' when generating HTML from a DOMDocument.

This primarily occurs when modifying article content.  If 'saveXML' is
used following 'loadHTML' there is the possibility of strangeness, such
as a self-closing anchor tag.

Note that the DOMDocument used in 'classes/feeditem/atom.php' came from
'loadXML', but we use 'saveHTML' since we're returning HTML content.