fix BinData#tojson with subtype 3

This is a fix for problem with BinData UUID's of subtype 3.

You can reproduce it with the following command:
```
Sheldor(mongod-2.4.3) dev-comp-po> db.foo.findOne();
Thu May 30 13:57:46.863 JavaScript execution failed: ReferenceError: uuidType is not defined at /Users/pstadler/.mongorc.js:L390
```

It know correctly falls back to the configured `uuid_type`.