This may cause a problem with MongoClient's constructor. After parsing the server string, but before processing the options array, we apply defaults:

```
if (link->servers->options.default_w == -1) {
	if (bc) {
		/* Default to WriteConcern=0 for Mongo */
		link->servers->options.default_w = 0;
	} else {
		/* Default to WriteConcern=1 for MongoClient */
		link->servers->options.default_w = 1;
	}
}
```