htmlbalance: be compatible with HTML::Tree 4.0

The HTML::Tree changelog says:

    [THINGS THAT MAY BREAK YOUR CODE OR TESTS]
    ...
    * Attribute names are now validated in as_XML and invalid names will
      cause an error.

and indeed the regression tests do get an error.

IkiWiki/Plugin/htmlbalance.pm

@@ -43,7 +43,7 @@ sub sanitize (@) {
 	my @nodes = $tree->disembowel();
 	foreach my $node (@nodes) {
 		if (ref $node) {
-			$ret .= $node->as_XML();
+			$ret .= $node->as_HTML(undef, '', {});
 			chomp $ret;
 			$node->delete();
 		}