Add a function to make html fulltext galley validate
Grab the contents of the body-element of a html-document, wrap it in a div, and return it as a string.
The returned string is a html fragment where empty tags will be self closed, as required by xhtml.
This function requires the DOMDocument module to operate. If DOMDocument module is not available,
a warning in form of a html-comment is added to the html string.