Merge pull request AFNetworking#4206 from Huang-Libo/patch-1

Update AFAutoPurgingImageCache.m

UIKit+AFNetworking/AFAutoPurgingImageCache.m

@@ -28,7 +28,7 @@
 @interface AFCachedImage : NSObject
 
 @property (nonatomic, strong) UIImage *image;
-@property (nonatomic, strong) NSString *identifier;
+@property (nonatomic, copy) NSString *identifier;
 @property (nonatomic, assign) UInt64 totalBytes;
 @property (nonatomic, strong) NSDate *lastAccessDate;
 @property (nonatomic, assign) UInt64 currentMemoryUsage;