Update the convenient API for UIImage (not SDAnimatedImage) to grab the current image format when no encode format provided

This match two classes behavior

SDWebImage/Core/UIImage+MultiFormat.m

@@ -9,6 +9,7 @@
 #import "UIImage+MultiFormat.h"
 #import "SDImageCodersManager.h"
 #import "SDAnimatedImageRep.h"
+#import "UIImage+Metadata.h"
 
 @implementation UIImage (MultiFormat)
 
@@ -41,7 +42,7 @@
         }
     }
 #endif
-    return [self sd_imageDataAsFormat:SDImageFormatUndefined];
+    return [self sd_imageDataAsFormat:self.sd_imageFormat];
 }
 
 - (nullable NSData *)sd_imageDataAsFormat:(SDImageFormat)imageFormat {