content attribute takes priority over the datetime attribute in RDFa

This change in RDFa was decided shortly after the release of EasyRdf beta1: https://github.com/rdfa/rdfa-website/commit/d4efc1383097aab06953beba9e1af826e6203b03

While fixing this, I notice it require another ```$content === null``` to be added to the step 11, which included quite a few of these already. I decided to move the content attribute processing higher up to avoid repeating these ```$content === null```.