Rework client interface

client/rpc_message.go

@@ -0,0 +1,27 @@
+package client
+
+type message struct {
+	topic       string
+	contentType string
+	payload     interface{}
+}
+
+func newMessage(topic string, payload interface{}, contentType string) Message {
+	return &message{
+		payload:     payload,
+		topic:       topic,
+		contentType: contentType,
+	}
+}
+
+func (m *message) ContentType() string {
+	return m.contentType
+}
+
+func (m *message) Topic() string {
+	return m.topic
+}
+
+func (m *message) Payload() interface{} {
+	return m.payload
+}