XEP-0016 error messages should set correct from attribute (the sender).

See XEP-0016, example 50.

src/java/org/jivesoftware/openfire/OfflineMessageStrategy.java

@@ -99,6 +99,7 @@ public class OfflineMessageStrategy extends BasicModule implements ServerFeature
             if (list != null && list.shouldBlockPacket(message)) {
                 Message result = message.createCopy();
                 result.setTo(message.getFrom());
+                result.setFrom(message.getTo());
                 result.setError(PacketError.Condition.service_unavailable);
                 XMPPServer.getInstance().getRoutingTable().routePacket(message.getFrom(), result, true);
                 return;