Fix of DBRef resaving/deserialization weird behaviour

[adamansky]

Using mongodb driver with native deserializer i've descovered strange bug:
1 Object created and saved with DBRef field
2 Then loaded (deserilizer processed their fields) and saved again.
3 After saving DBRef field became corrupted! IMHO due to $db field value (empty string) in DBRef //see patch

I've created test case and fix.

PS.
All test cases checked agains pure BSON serializer! Regardless of 'native_parser' option value!
Due to small typo:

var client = new Db(MONGODB, 
new Server("127.0.0.1", 27017, {auto_reconnect: true, poolSize: 4, 
 native_parser: (process.env['TEST_NATIVE'] != null) ? true : false}));

But correct:

var client = new Db(MONGODB, 
new Server("127.0.0.1", 27017, {auto_reconnect: true, poolSize: 4}), 
{native_parser: (process.env['TEST_NATIVE'] != null) ? true : false});