title: cellForItemAtIndexPath: is not called when using custom UICollectionViewLayout subclass
body: <p>When using my custom <code>UICollectionViewLayout</code> subclass, <code>cellForItemAtIndexPath:</code> is not called (I checked using breakpoints and debug output). This is how i use the custom layout:</p>
  
  <pre><code>- (void)viewDidLoad
      {
          [super viewDidLoad];
  
          DMSGridLayout* gridLayout = [[DMSGridLayout alloc] init];
  
  //      UICollectionViewFlowLayout* flow = [[UICollectionViewFlowLayout alloc] init];
  //      [flow setItemSize:CGSizeMake(150, 150)];
  
          UICollectionView *collection = [[UICollectionView alloc] initWithFrame:self.view.bounds collectionViewLayout:gridLayout];
          collection.delegate = self;
          collection.dataSource = self;
          self.collectionView = collection;
  
          [self.view addSubview:self.collectionView];
          [self.collectionView registerClass:[DMSGridCell class] forCellWithReuseIdentifier:@"CollectionCell"];
      }
  </code></pre>
  
  <p>But when i change the above code to use <code>UICollectionViewFlowLayout</code> instead of my custom subclass, <code>cellForItemAtIndexPath:</code> is called.</p>
  
  <p>Could it be that some code from my custom layout subclass prevents <code>cellForItemAtIndexPath</code> from being called?</p>