make topic impl return their names (easier to dbg)

src/main/java/org/springframework/data/redis/listener/ChannelTopic.java

@@ -72,4 +72,9 @@ public class ChannelTopic implements Topic {
 		}
 		return true;
 	}
+
+	@Override
+	public String toString() {
+		return channelName;
+	}
 }

src/main/java/org/springframework/data/redis/listener/PatternTopic.java

@@ -62,4 +62,9 @@ public class PatternTopic implements Topic {
 		}
 		return true;
 	}
+
+	@Override
+	public String toString() {
+		return channelPattern;
+	}
 }