Use Header.Set() to override the existing header value (#1032)

Header.Add() appends string if there are same header key exists. Use Header.Set() to prevent this.

mqtrigger/messageQueue/kafka.go

@@ -133,7 +133,7 @@ func kafkaMsgHandler(kafka *Kafka, producer sarama.SyncProducer, trigger *crd.Me
 	}
 
 	for k, v := range headers {
-		req.Header.Add(k, v)
+		req.Header.Set(k, v)
 	}
 	// Make the request
 	var resp *http.Response