title: UICollectionViewCell is overlapped when scrolling
body: <p>Following code is a very simple program which is used to display numbers from 1-10000 in the <code>UICollectionView</code>. It is displaying correctly without scrolling, but the cells are overlapped if you scroll down and scroll back the collection view.</p>
  
  <pre><code>-(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView{
      return 1;
  }
  -(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{
      return 10000;
  }
  -(CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath{
      return CGSizeMake(100,30);
  }
  -(UICollectionViewCell*)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
      NSString *identifier = @"cell_id";
      UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:identifier forIndexPath:indexPath];
  
      UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 100, 30)];
      [label setText:[NSString stringWithFormat:@"%d", indexPath.item, nil]];
      [cell.contentView addSubview:label];
  
      return cell;
  }
  </code></pre>