Attributes starting with data-xwiki- aren't preserved during XHTML parsing

The XHTML parser removes attributes starting with data-xwiki- on all group (<div>) and format-tags (like <strong>, <em> and <span>) even if they aren't recognized as metadata tags. This means that attributes starting with data-xwiki- aren't preserved on syntax resulting in these tags when using the WYSIWYG editor. Instead, those attributes should only be removed when the element is actually recognized as metadata.