[ASDisplayNode] Placeholders should always be recreated if returning to past nodes.

If previously-displayed contents is gone (e.g. clearContents), and is not finished displaying
by the time the node is onscreen, recreate the placeholder immediately.

AsyncDisplayKit/ASMultiplexImageNode.mm

@@ -265,6 +265,11 @@ typedef void(^ASMultiplexImageLoadCompletionBlock)(UIImage *image, id imageIdent
   }
 }
 
+- (BOOL)placeholderShouldPersist
+{
+  return (self.image == nil && self.imageIdentifiers.count > 0);
+}
+
 /* displayWillStart in ASNetworkImageNode has a very similar implementation. Changes here are likely necessary
  in ASNetworkImageNode as well. */
 - (void)displayWillStart