title: rac_signalForSelector: needs empty implementation
body: <p>I have a class that implements the UICollectionViewDelegate protocol.</p>
  
  <p>I'm using rac_signalForSelector: to register selection like this:</p>
  
  <pre><code>[self rac_signalForSelector:@selector(collectionView:didSelectItemAtIndexPath:)]
  </code></pre>
  
  <p>but it only fires if I leave in an empty implementation of collectionView:didSelectItemAtIndexPath: like this:</p>
  
  <pre><code>- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath {
      // Empty
  }
  </code></pre>
  
  <p>I tried with rac_signalForSelector:fromProtocol: as well without luck.</p>
  
  <p>I'm using Reactive Cocoa 2.2.4</p>
  
  <p>Thanks, 
  Mikkel</p>