title: _.map not working correctly
body: <pre><code>var models = require('../models')
    , _ = require('underscore')
    , Restaurant = models.restaurant
    , RestaurantBranch = models.restaurant_branch;
  
  
  module.exports = {
    index: function (req, res) {
      var title = 'Restaurants near you';
  
      RestaurantBranch.find({country: 'Ghana', region: 'Greater Accra'}, function (err, branches) {
  
  
        var results = _.map(branches, function (branch) {
          Restaurant.findById(branch._restaurantId, function (err, restaurant) {
            return {
              'restaurant': restaurant,
              'branch': branch
            };
          });
        });
  
        res.send(results);
      });
  
    }
  };
  </code></pre>
  
  <p>I am having a problem getting _.map to work the way I want. Instead of getting a new array with objects <code>{restaurant: restaurant, branch: branch}</code>. I get <code>[null, null]</code> instead.</p>
  
  <p>I have tried lodash instead of underscore and I get the same behavior.</p>