Recent minor version update introduced a breaking change: queries which contain properties which value is 'undefined' cause an error: MongoError "cannot compare to undefined"

previously undefined was treated the same was as null ( for as long as I can remember )

I understand that previous behaviour might have been an unintended feature, however the introduced fix, changes behaviour in a way that will definitely break many applications.

here's a coffeescript example of code which used to work in versions < 2.2.13, and which now throws:

```
docs = [

]

db.events.insert docs
.then ->

Promise.all [
db.events.findOne a : undefined
db.events.findOne a : null
]

.then ( [ result_for_undefined, result_for_null ] ) ->

console.log result_for_undefined._id.equals result_for_null._id
```

the call to console.log would print 'true'