HMTL Cleaner should not expand some empty elements (BR, HR, etc) in order to support IE6

IE6 doesn't understand

<br></br>

but it works with

<br/>

. Same for HR.

Full list: "area", "base", "br", "col", "hr", "img", "input", "link", "meta", "p", "param"

However for TEXTAREA and SCRIPT we need to keep the expanded notation since otherwise it'll fail. See XWIKI-3767