title: Can't use $multiply with Number
body: <p>I am running into an update problem using Mongoose, below is the schema definition.  For example sake, below I would like to udpate the price of a car by multiplying the number of tires by 500:</p>
  
  <p>car.js = 
  <div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>var mongoose = require('mongoose');
  
  module.exports = mongoose.model('car', {
    
    make     : {type:String},
    model    : {type:String},
    num_tires: {type:Number, default:0}
    price    : {type:Number, default:0}
    
  });
    </code></pre>
  </div>
  </div>
  </p>
  
  <p>updateCost.js =</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>var Car = require('car');
  
  Car.update(
    {make: 'Honda'},
    {price: {$multiply: ['$num_tires', 500]}},
    {multi: true},
    function(err) {
      console.log("Thar's an err", err);
    });</code></pre>
  </div>
  </div>
  </p>
  
  <p>The error I am receiving is: "Can't use $multiply with Number".</p>
  
  <p>Is there a way around the schema definition Number to update the price?  Thanks everyone for their time.</p>