title: Enable bounce/scroll for UICollectionView while no cells
body: <p>Due to some purposes(like pull to refresh), I need a UICollectionView can bounce or scroll when there is no cells -- means <code>numberOfItemsInSection:</code> return 0</p>
  
  <p>I have my code like this:</p>
  
  <pre><code>...
          UICollectionViewFlowLayout *flowLayout =[[UICollectionViewFlowLayout alloc] init];
          flowLayout.scrollDirection = UICollectionViewScrollDirectionVertical;
          flowLayout.minimumInteritemSpacing = SPLIT_SPACE;
          flowLayout.minimumLineSpacing = SPLIT_SPACE;
          targetCollection = [[UICollectionView alloc] initWithFrame:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height - TABBAR_HEIGHT)
                                                      collectionViewLayout:flowLayout];
          [targetCollection registerNib:[UINib nibWithNibName:@"DashboardCell" bundle:[NSBundle mainBundle]] forCellWithReuseIdentifier:@"DashboardCell"];
  
          targetCollection.autoresizingMask = UIViewAutoresizingFlexibleHeight;
          targetCollection.backgroundColor = [UIColor colorWithHex:@"#EAEAEA"];
          targetCollection.contentInset = UIEdgeInsetsMake(0, 0, 20, 0);
          targetCollection.alwaysBounceVertical = YES; 
  ...
      #pragma mark - UICollectionViewDataSource
      - (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section;
      {
          return 0;
      }
  </code></pre>
  
  <p>However, when testing, this empty UICollectionView cannot bounce nor scroll. I suspect it is related to empty cell, but I do need to enable the bounce or scroll when no cell. 
  This is similar to another of my problem:<a href="https://stackoverflow.com/questions/28940364/svpulltorefresh-cannot-pull-on-an-empty-uicollectionview">SVPullToRefresh cannot pull on an empty UICollectionView</a></p>