If nil is given as URL to setImageWithURL:, set the image to the placeholder image (or nil if no placeholder)

This is the recommanded way clear an image for reuse (as a UITableViewCell for instance). Calling setImageWithURL:
with nil URL will have the effect to cancel an eventual currently in progress download of a thumbnail for this UIImageView.
Affecting the image property directly won't have this effect and may lead to image being re-affected once download is completed.

UIImageView+WebCache.m

@@ -28,6 +28,10 @@
     {
         cachedImage = [manager imageWithURL:url];
     }
+    else
+    {
+        self.image = placeholder;
+    }
 
     if (cachedImage)
     {