title: How are cells deselected programmatically in UICollectionView when allowMultipleSelection is enabled?
body: <p>I have allowMultipleSelection enabled in a collection view. The cells change to and from their 
   selected states when tapped. All good. However, when I want to reset the entire view to selected state:NO using the code below, the cells appears to be entirely deselected until I then make a new selection, at which point all previously selected cells show their previously selected state.</p>
  
  <p>i.e. despite appearances the collectionview is not updating it's current selection list when I programmatically deselect the cells</p>
  
  <pre><code>- (void)clearCellSelections {
     for (LetterCell  *cell in self.collectionView.visibleCells) {
          [cell prepareForReuse];
      }
  }
  </code></pre>
  
  <p>In custom cell:</p>
  
  <pre><code>- (void)prepareForReuse
  {
      [super prepareForReuse];
      [self setSelected:NO];
  }
  </code></pre>
  
  <p>What am I doing wrong? Is there another way to deselect all cells?</p>
  
  <p>Thanks TBlue for taking a look</p>