Coercing docElm into a boolean

At least with Chrome, isSupported can return the requestFullScreen function instead of a boolean.

[This Plunker](http://plnkr.co/edit/YLYpfHrH8woIEsxlctun?p=preview) illustrates that currently isSupported returns a pointer to a (possibly vendor-prefixed) requestFullscreen function.

[This Plunker](http://plnkr.co/edit/1X8jys4Ev94Q7yKNa7jo?p=preview) illustrates that the change outputs only true/false values.