Fix associated object bug in UIKit extensions (#3872)

UIKit+AFNetworking/UIButton+AFNetworking.m

@@ -103,11 +103,11 @@ static const char * af_backgroundImageDownloadReceiptKeyForState(UIControlState
 
 + (AFImageDownloader *)sharedImageDownloader {
 
-    return objc_getAssociatedObject(self, @selector(sharedImageDownloader)) ?: [AFImageDownloader defaultInstance];
+    return objc_getAssociatedObject([UIButton class], @selector(sharedImageDownloader)) ?: [AFImageDownloader defaultInstance];
 }
 
 + (void)setSharedImageDownloader:(AFImageDownloader *)imageDownloader {
-    objc_setAssociatedObject(self, @selector(sharedImageDownloader), imageDownloader, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
+    objc_setAssociatedObject([UIButton class], @selector(sharedImageDownloader), imageDownloader, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
 }
 
 #pragma mark -

UIKit+AFNetworking/UIImageView+AFNetworking.m

@@ -48,11 +48,11 @@
 @implementation UIImageView (AFNetworking)
 
 + (AFImageDownloader *)sharedImageDownloader {
-    return objc_getAssociatedObject(self, @selector(sharedImageDownloader)) ?: [AFImageDownloader defaultInstance];
+    return objc_getAssociatedObject([UIImageView class], @selector(sharedImageDownloader)) ?: [AFImageDownloader defaultInstance];
 }
 
 + (void)setSharedImageDownloader:(AFImageDownloader *)imageDownloader {
-    objc_setAssociatedObject(self, @selector(sharedImageDownloader), imageDownloader, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
+    objc_setAssociatedObject([UIImageView class], @selector(sharedImageDownloader), imageDownloader, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
 }
 
 #pragma mark -