The issue was raised in #7628, where an anchor tag was being changed from
<a name="anchor"></a> to <a name="anchor"/> by SS_HTMLValue, when
HtmlEditorField::saveInto() parses the HTML fragments.
This is because SS_HTMLValue uses DOMDocument::saveXML(), which is fine
for saving an XML document, but not suitable for HTML. This fix changes
that to use DOMDocument::saveHTML() instead.
Note that we can't use the parameter to saveHTML() for selecting a single
node only, as that's only supported in PHP 5.3.6+, SilverStripe 3.0 supports
PHP 5.3.2 as a minimum. The workaround for this shortcoming is to replace
unncessary output by DOMDocument with a regular expression.
