fix skiping fields in json response which have zero value

[komly]

After this changes default json marshaler won't skip fields with zero values in response body.

ts := &types.Timeslot{
    Start: 0,
    Duration: 2,
}

now we will response with:

{"start":0,"duration":2}

instead of

{"duration":2}

[utrack]

It breaks proto convention and current behaviour. Servers can control it via OverrideMarshaler("application/json",MarshalerPbJSON{ ... EmitDefaults: true ...}).