Fix alpha handling in image decoder (fix #204)

SDWebImage/SDWebImageDecoder.m

@@ -104,8 +104,8 @@ static SDWebImageDecoder *sharedInstance;
                               alphaInfo != kCGImageAlphaNoneSkipFirst &&
                               alphaInfo != kCGImageAlphaNoneSkipLast);
 
-    int bytesPerPixel = imageHasAlphaInfo ? 4 : 3;
-    CGBitmapInfo bitmapInfo = imageHasAlphaInfo ? kCGImageAlphaPremultipliedLast : kCGImageAlphaNone;
+    int bytesPerPixel = alphaInfo != kCGImageAlphaNone ? 4 : 3;
+    CGBitmapInfo bitmapInfo = imageHasAlphaInfo ? kCGImageAlphaPremultipliedLast : alphaInfo;
 
     CGContextRef context = CGBitmapContextCreate(NULL,
                                                  CGImageGetWidth(imageRef),