unpack don't merge sub-messages

[GoogleCodeExporter]

According to google protobuf documentation:
https://developers.google.com/protocol-buffers/docs/encoding#optional

"... For embedded message fields, the parser merges multiple instances of the 
same field, as if with the Message::MergeFrom method – that is, all singular 
scalar fields in the latter instance replace those in the former, singular 
embedded messages are merged, and repeated fields are concatenated ..."

But protobuf-c replaces submessage field with last value.

What steps will reproduce the problem?
1. Source
test.proto:
----------
message MergeTest {
  message SubMessage {
    optional uint32 f1 = 1;
    optional uint32 f2 = 2;
  }
  optional SubMessage submsg = 5;
}
----------

test.c:
----------
#include <stdio.h>
#include <stdlib.h>

#include "test.pb-c.h"

uint8_t MERGE_TEST[11] = { 0x2a,0x4,0x8,0xa,0x10,0x2a,0x2a,0x3,0x8,0x89,0x6 };
/*
MergeTest(submsg=SubMessage(f1=10, f2=42)) + 
MergeTest(submsg=SubMessage(f1=777))
*/

int main() {
  MergeTest* m = merge_test__unpack(NULL, sizeof(MERGE_TEST), MERGE_TEST);
  printf("m->submsg->f1: %d\nm->submsg->f2: %d\n", m->submsg->f1, m->submsg->f2);
}
----------

2. Compile and execute

What is the expected output? What do you see instead?
----------
m->submsg->f1: 777
m->submsg->f2: 0
----------

instead of f1=777,f2=42

What version of the product are you using? On what operating system?
protobuf-c 0.15

Please provide any additional information below.

Original issue reported on code.google.com by r.lomono...@gmail.com on 16 Nov 2012 at 6:28