MarshalJSON should be defined on T not *T

Otherwise "%v" format would be used by json.Marshal(T).

password_message.go

@@ -40,7 +40,7 @@ func (src *PasswordMessage) Encode(dst []byte) []byte {
 }
 
 // MarshalJSON implements encoding/json.Marshaler.
-func (src *PasswordMessage) MarshalJSON() ([]byte, error) {
+func (src PasswordMessage) MarshalJSON() ([]byte, error) {
 	return json.Marshal(struct {
 		Type     string
 		Password string