Remove pointless condition in keep content check.
Remove pointless condition in keep content check.

It used to check for the presence of a currentNode.nodeName.toLowerCase
property. Not sure why, but this MUST always exist otherwise an error would
have been thrown at line 316 when currentNode.nodeName.toLowerCase() was called,
since currentNode has not been reassigned or significatnly changed since then.

voku/DOMPurify,neilj/DOMPurify,togusafish/t-ashula-_-DOMPurify,kwangkim/DOMPurify,ConradIrwin/DOMPurify,tdeekens/DOMPurify,Joris-van-der-Wel/DOMPurify,Joris-van-der-Wel/DOMPurify,togusafish/t-ashula-_-DOMPurify,Joris-van-der-Wel/DOMPurify,ConradIrwin/DOMPurify,kwangkim/DOMPurify,tdeekens/DOMPurify,voku/DOMPurify,neilj/DOMPurify,tdeekens/DOMPurify,neilj/DOMPurify