Do not encode undefined fields

There is probably no sense in wiring undefined fields in messages. Actually doing so breaks things up on Riak server:

```
error:data_error:[{zlib,call,3,[]},
 {zlib,inflate,2,[]},
 {zlib,unzip,1,[]},
 {riak_object,
 decode_vclock,2,
 [{file,
 "src/riak_object.erl"},
 {line,798}]},............
```

Of course you can avoid sending undefined keys in application but its really easier to fix the problem in protobuf.js