Fallback to non-skeleton Header (#416)

Sometimes my collectionview's header doesn't need to be 'skeletoned' while the content does.  

When  collectionSkeletonView(_ skeletonView: UICollectionView, supplementaryViewIdentifierOfKind: String, at indexPath: IndexPath) -> ReusableCellIdentifier? returns nil, it's best if we fallback to the non-skeleton header.

Sources/Collections/SkeletonCollectionDataSource.swift

@@ -107,7 +107,7 @@ extension SkeletonCollectionDataSource: UICollectionViewDataSource {
             return view
         }
 
-        return UICollectionReusableView()
+        return originalCollectionViewDataSource?.collectionView?(collectionView, viewForSupplementaryElementOfKind: kind, at: indexPath) ?? UICollectionReusableView()
     }
 
 }