title: Mongo findAndModify returns null
body: <p>Following.follow = function(id1, id2, cb) {
        console.log(id1) // returns Matt
        console.log(id2) // returns Simone</p>
  
  <pre><code>  Following.collection.findAndModify({
        query: {
            ownerId: id1
        },
        update: {
            $addToSet: {
                followedBy: id2
            }
        },
        upsert: true,
        new: true
    }, function(err, result, lastErrorObject) {
        cb(err, result)
        console.log(result) // returns null
    })
  }
  </code></pre>
  
  <p>I'm running tests with Mocha and my findAndModify functions will only return null. I read the documentation and can't seem to figure out what I'm doing wrong.  Upsert combined with true should make the document if it isn't found and new should return the modified object.</p>