feat(ios): cancel image load when unmounted (DylanVann#787)

Author: numandev1

ios/FastImage/FFFastImageView.m

@@ -1,5 +1,6 @@
 #import "FFFastImageView.h"
 #import <SDWebImage/UIImage+MultiFormat.h>
+#import <SDWebImage/UIView+WebCache.h>
 
 @interface FFFastImageView()
 
@@ -228,5 +229,8 @@ - (void)downloadImage:(FFFastImageSource *) source options:(SDWebImageOptions) o
                     }];
 }
 
-@end
+- (void)dealloc {
+    [self sd_cancelCurrentImageLoad];
+}
 
+@end