When jquery-timesince was first released, $.fn.attr() would look at

both attributes and properties, but this has since changed. If we have a

<time> element with datetime set as a property and not as an

attribute, it would come up empty.
This will now check the attribute first, for backwards-compatibility,

and fall back on trying to access it from the property.

Verified this fixed an issue with a manually-constructed <time> element.