add webp accepted content type only if WebP enabled

Send WebP as accepted content type only if it is enabled. In case it's not, stick with the standard formats in order to avoid the server sending us back WebP images.

@@ -67,7 +67,11 @@ static NSString *const kCompletedCallbackKey = @"completed";
         _downloadQueue = [NSOperationQueue new];
         _downloadQueue.maxConcurrentOperationCount = 6;
         _URLCallbacks = [NSMutableDictionary new];
-        _HTTPHeaders = [NSMutableDictionary dictionaryWithObject:@"image/webp,image/*;q=0.8" forKey:@"Accept"];
+#ifdef SD_WEBP
+        _HTTPHeaders = [@{@"Accept": @"image/webp,image/*;q=0.8"} mutableCopy];
+#elseif
+        _HTTPHeaders = [@{@"Accept": @"image/*;q=0.8"} mutableCopy];
+#endif
         _barrierQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderBarrierQueue", DISPATCH_QUEUE_CONCURRENT);
         _downloadTimeout = 15.0;
     }