title: Mongoose.js instance.save() callback not firing
body: <pre><code>var mongo = require('mongoose');
  var connection = mongo.createConnection('mongodb://127.0.0.1/test');
  
  connection.on("error", function(errorObject){
    console.log(errorObject); 
    console.log('ONERROR');
  });
  
  var Schema = mongo.Schema;
  var BookSchema = new Schema({ title : {type : String, index : {unique : true}}});
  var BookModel = mongo.model('abook', BookSchema);
  var b = new BookModel({title : 'aaaaaa'});
  
  b.save( function(e){ 
    if(e){
      console.log('error')
    }else{
      console.log('no error')
  }});
  </code></pre>
  
  <p>Neither the 'error', or 'no error' are printed to the terminal. What's more the connection.on 'error' doesn't seem to fire either. I have confirmed that MongoDb is running.</p>