MongoDB 2.6: Ensure $in argument is a real BSON array

This PR targets the same error as the first commit in doctrine/mongodb-odm#741

When doing a custom query with doctrine against MongoDB 2.6 and using the QueryBuilder this exception is thrown:
```MongoCursorException: localhost:27017: Can't canonicalize query: BadValue $in needs an array```

I'm not sure if it is the right place for the array_values() call, but you should see what I tried to fix with this. :)