Fix fatal error with activity assignee and anonymous users

Here is a patch to fix this issue:
http://drupal.org/node/1258320

I tracked it down to sendActivityCopy() in the file Case.php, where it simply assumes the acting user is logged in while the activity is being created. This patch allows it to default to the activity's creator in the event that the acting user is anonymous (as is the case with many webform submissions).