fix MessageCodec Decode

[ramonberrutti]

Inside bson decode the code is doing something different to Encode:

	rval := reflect.ValueOf(val)
	switch rval.Kind() {
	case reflect.Ptr:
		if rval.IsNil() {
			return ErrDecodeToNil
		}
		rval = rval.Elem()
	case reflect.Map:
		if rval.IsNil() {
			return ErrDecodeToNil
		}
	default:
		return fmt.Errorf("argument to Decode must be a pointer or a map, but got %v", rval)
	}

[vallahaye]

Hello Ramón,

Apologies for the late response.

The last time I used protobson, the encoding/decoding of Protobuf messages worked correctly. I'm not aware of any change in the Mongo driver that has broken anything. The API used by protobson is public and the driver is still in v1.

That being said, I suspect you're passing a nil pointer to the struct you want to deserialize into, in which case you need to initialize it before decoding:

example := new(examplepb.Example)
collection.FindOne(...).Decode(example)

Or:

var example examplepb.Example
collection.FindOne(...).Decode(&example)

If it is not the case, could you please create an issue related to that PR and give me more context around your contribution, i.e. what's wrong with the current implementation.

Have a nice day