[Issue #1115] Only set placeholder image if not nil

AFNetworking/UIImageView+AFNetworking.m

@@ -109,7 +109,9 @@ static char kAFImageRequestOperationObjectKey;
 
         self.af_imageRequestOperation = nil;
     } else {
-        self.image = placeholderImage;
+        if (placeholderImage) {
+            self.image = placeholderImage;
+        }
 
         AFImageRequestOperation *requestOperation = [[AFImageRequestOperation alloc] initWithRequest:urlRequest];
         [requestOperation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {