Add the test case for cache serializer feature

Tests/Tests/SDWebImageManagerTests.m

@@ -157,6 +157,34 @@
     [self waitForExpectationsWithCommonTimeout];
 }
 
+- (void)test10ThatCacheSerializerWork {
+    XCTestExpectation *expectation = [self expectationWithDescription:@"Cache serializer work"];
+    NSURL *imageURL = [NSURL URLWithString:kTestJPEGURL];
+    __block NSData *imageData;
+    
+    SDWebImageCacheSerializer *cacheSerializer = [SDWebImageCacheSerializer cacheSerializerWithBlock:^NSData * _Nullable(UIImage * _Nonnull image, NSData * _Nullable data, NSURL * _Nullable imageURL) {
+        imageData = [image sd_imageDataAsFormat:SDImageFormatPNG];
+        return imageData;
+    }];
+    
+    SDWebImageManager *manager = [[SDWebImageManager alloc] initWithCache:[SDImageCache sharedImageCache] loader:[SDWebImageDownloader sharedDownloader]];
+    manager.cacheSerializer = cacheSerializer;
+    // Check download and store custom disk data
+    [[SDImageCache sharedImageCache] removeImageForKey:kTestJPEGURL withCompletion:^{
+        [manager loadImageWithURL:imageURL options:0 progress:nil completed:^(UIImage * _Nullable image, NSData * _Nullable data, NSError * _Nullable error, SDImageCacheType cacheType, BOOL finished, NSURL * _Nullable imageURL) {
+            // Dispatch to let store disk finish
+            dispatch_after(dispatch_time(DISPATCH_TIME_NOW, kMinDelayNanosecond), dispatch_get_main_queue(), ^{
+                NSData *diskImageData = [[SDImageCache sharedImageCache] diskImageDataForKey:kTestJPEGURL];
+                expect(diskImageData).equal(imageData); // disk data equal to serializer data
+                
+                [expectation fulfill];
+            });
+        }];
+    }];
+    
+    [self waitForExpectationsWithCommonTimeout];
+}
+
 - (NSString *)testJPEGPath {
     NSBundle *testBundle = [NSBundle bundleForClass:[self class]];
     return [testBundle pathForResource:@"TestImage" ofType:@"jpg"];