Type error

Hi,

use PR "min-max-key" to handle this case: https://github.com/twillouer/fongo/blob/e3c3f018964aa817cc974beb5833568b98360655/src/test/java/com/foursquare/fongo/FongoFindTest.java

```java
    DBCollection collection = fongoRule.newCollection();
    collection.insert(new BasicDBObject("_id", 1).append("field", 12L));
    DBObject result = collection.findOne(new BasicDBObject("field", 12));
    assertEquals(new BasicDBObject("_id", 1).append("field", 12L), result);
```

(differents types when searching into fields)