Added missing background transform to TinyPNG convert call

Author: Paul Manzotti

src/TinyPNG/Extensions/ConvertExtensions.cs

@@ -33,7 +33,16 @@ public static async Task<TinyPngConvertResponse> Convert(this Task<TinyPngCompre
 
         TinyPngCompressResponse compressResponse = await result;
 
-        string requestBody = JsonSerializer.Serialize(new { convert = new { type = convertOperation } }, TinyPngClient._jsonOptions);
+        string requestBody = JsonSerializer.Serialize(new {
+            convert = new
+            {
+                type = convertOperation
+            },
+            transform = new
+            {
+                background = backgroundTransform
+            }
+        }, TinyPngClient._jsonOptions);
 
         HttpRequestMessage msg = new(HttpMethod.Post, compressResponse.Output.Url)
         {