title: Angular $resource update method not found
body: <p>I'm following the Angular main page and I've extended $resource to add the update method like this:</p>
  
  <pre><code>angular.module('user', ['ngResource']).
  factory('User', function($resource) {
    var User= $resource('/user/:id', {
      update: {
        method: 'PUT'
      }
    });
  
    User.prototype.update = function(cb) {
      return User.update({  // this line throws the error
        id: this.id
      }, angular.extend({}, this, {
        _id: undefined
      }), cb);
    };
  </code></pre>
  
  <p>However running:</p>
  
  <pre><code>$scope.user.update()
  </code></pre>
  
  <p>throws a <strong>TypeError: Object function h(a){v(a||{},this)} has no method 'update'</strong></p>
  
  <p>I can't see what I'm missing right now, any help appreciated</p>