title: findAndModify - MongoError: exception: must specify remove or update
body: <p>Id like to update an array and return the doc.  Is my findAndModify syntax correct? </p>
  
  <pre><code>this.becomeFollower = function(title, username, callback){
      "use strict"
  
      posts.findAndModify({
          query: {"title":title, "roster":"yes"},
          update: { "$addToSet": { "followers":username } },
          new: true,
          upsert: true
          }, 
          function(err, doc){
              console.log('find and modified  ' +doc);
          });
  
  }
  </code></pre>
  
  <p>I had no problem using this:</p>
  
  <pre><code>    posts.update({"title":title, "roster":"yes"}, { "$addToSet": { "followers":username } }, function(err, roster){
          "use strict"
          if(err) return callback(err, null);
          callback(err, roster);
      });
  </code></pre>