title: Sorting in Mongoose doesnt work correctly
body: <p>Sorting doesnt work with Mongoose, but I don't understand how this can be possible since I've searched so much! Any time I tried it didn't work. Here is my code</p>
  
  <pre><code>Pics.find({}).limit(8).populate('creator').sort("created_at", 1).execFind(function(err, docs){
      console.log(docs);
  });
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>TypeError: Invalid sort() argument. Must be a string or object.
  at Query.sort (/Users/n/Proj/feetshot/node_modules/mongoose/lib/query.js:755:11)
  at /Users/n/Proj/feetshot/app.js:112:45
  at callbacks (/Users/n/Proj/feetshot/node_modules/express/lib/router/index.js:272:11)
  at param (/Users/n/Proj/feetshot/node_modules/express/lib/router/index.js:246:11)
  at pass (/Users/n/Proj/feetshot/node_modules/express/lib/router/index.js:253:5)
  at Router._dispatch (/Users/n/Proj/feetshot/node_modules/express/lib/router/index.js:280:4)
  at Object.handle (/Users/n/Proj/feetshot/node_modules/express/lib/router/index.js:45:10)
  at Context.next (/Users/n/Proj/feetshot/node_modules/express/node_modules/connect/lib/http.js:204:15)
  at Context.&lt;anonymous&gt; (/Users/n/Proj/feetshot/node_modules/passport/lib/passport/context/http/actions.js:64:8)
  at SessionStrategy.pass (native)
  </code></pre>
  
  <p>How can I make sort working?</p>
  
  <p>Here is my schema:</p>
  
  <pre><code>var PicSchema = new Schema({
  creator: { type: Schema.ObjectId, ref: 'Users' }
  , body: String
  , created_at: { type: Date, default: Date.now }
  , link: { type: String, index: { unique: true } }
  , twitter_id: { type: String, index: { unique: true } }
  , tags: [String]
    , likes: [{ type: Schema.ObjectId, ref: 'Users' }]
     });
  </code></pre>