Correct auto-0 public_ack.

Actually, public_ack can be NULL and that's ok.  It indicates that the
donor has not answered the question yet.

Supporters/lib/Supporters.pm

@@ -126,8 +126,6 @@ sub addSupporter ($$) {
 
   die "ledger_entity_id required" unless defined $sp->{ledger_entity_id};
 
-  $sp->{public_ack} = 0 if not defined $sp->{public_ack};
-
   if ($sp->{public_ack}) {
     die "display_name required if public_ack requested" unless defined $sp->{display_name};
   }