FIX Not calling updateFromNode on re-opening an image

Inside mediaform updateFromEditor we called $(this).updateFromNode, but this wasnt
pointing to anything useful after a refactor, which resulted in the fields not
being filled in with current data when editing an already-inserted image
Merge pull request #1145 from silverstripe-rebelalliance/bug/image_reedit_attr

FIX Not calling updateFromNode on re-opening an image