ugly fix for nil context

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

segmentio.go

@@ -177,9 +177,11 @@ func (k *kBroker) Publish(topic string, msg *broker.Message, opts ...broker.Publ
 		return err
 	}
 	kmsg := kafka.Message{Value: val}
+	if options.Context != nil {
 		if key, ok := options.Context.Value(publishKey{}).([]byte); ok && len(key) > 0 {
 			kmsg.Key = key
 		}
+	}
 
 	k.Lock()
 	writer, ok := k.writers[topic]