Pass CurrentUser to HandleAction methods

CurrentUser is often needed for mail actions, so pass
it as a parameter to HandleAction methods. Also makes
the code consistent with the Mail plugins documentation
which lists CurrentUser as a passed parameter.

lib/RT/Interface/Email.pm

@@ -248,6 +248,7 @@ sub Gateway {
             Action      => $action,
             Subject     => $Subject,
             Message     => $Message,
+            CurrentUser => $CurrentUser,
             Ticket      => $Ticket,
             TicketId    => $args{ticket},
             Queue       => $SystemQueueObj,