Fix the behavior of ShowTopicChange and SyncTopic (#1086)

Currently, the "topic_change" events are ignored if both, 
ShowTopicChange and SyncTopic are set, and forwarded otherwise. 

This pull requests changes the behavior such that the events are 
only forwarded if one of those two config options is set to true 
and ignored otherwise.

gateway/handlers.go

@@ -179,7 +179,7 @@ func (gw *Gateway) ignoreEvent(event string, dest *bridge.Bridge) bool {
 		}
 	case config.EventTopicChange:
 		// only relay topic change when used in some way on other side
-		if dest.GetBool("ShowTopicChange") && dest.GetBool("SyncTopic") {
+		if !dest.GetBool("ShowTopicChange") && !dest.GetBool("SyncTopic") {
 			return true
 		}
 	}