uniq(el) should return a string selectable with document.querySelector

Hi,

if `uniq(el)` has to recursively go up to `document`, the string it returns is not selectable because `document.querySelector('html:nth-child(1)')` returns `null`, whereas `document.querySelector('html')` works (tested in chrome).

Not sure if this should be in `indexof` ? It seems like the use case is specific to `uniq-selector`.