Add .hasOwnProperty checks, fixes #1223

As detailed on #1223, Firefox 27 started exposing a new HTMLPropertiesCollection interface on HTMLElement.
Since all elements on the DOM inherit from HTMLElement we can safely safeguard our code against confusing a plain object with an element by checking if `.properties` has been intentionally defined or it's inherited from the parent prototype.

I added the same check on `.content` just to remain on the safe side. We never know if/when this interface could also be added on HTMLElement (or poor code modifying `Object`'s prototype)