improve contenteditable test

I was annoyed that I couldn't get contenteditable checks on mobile
without UA sniffing,

So, it turns out, this is true - http://goo.gl/RJh7r (check it out on
ios <= 4, or android <= 2.3);

this is about an order of maginude  slower than a basic contentElement
in docElement, but it works in all mobile browsers correctly. =D