[Azure Search] Prevent ObjectDisposedException when Indexing using "M…

…erge" operation (Azure#7011)

* Fix the request content being disposed betweeen .NET Core and .Net framework

* Add SessionRecord for the new test

sdk/search/Microsoft.Azure.Search.Data/src/Customizations/Documents/DocumentsOperations.Customization.cs

@@ -290,7 +290,15 @@ await Client.DocumentsProxy.IndexWithHttpMessagesAsync(
                 HttpRequestMessage httpRequest = result.Request;
                 HttpResponseMessage httpResponse = result.Response;
 
+                // NOTE: It is not possible to read the http request's string content property here, 
+                // via .NET framework's HttpClient, as the Content gets disposed as soon as the request is sent.
+                // Thus the batch is re-serialized, which is what happens in the IndexWithHttpMessagesAsync() method
+                // In .NET Core, the Content doesn't get disposed, so it's safe to read the string content here.
+#if FullNetFx
+                string requestContent = Rest.Serialization.SafeJsonConvert.SerializeObject(batch, jsonSettings);
+#else
                 string requestContent = await httpRequest.Content.ReadAsStringAsync().ConfigureAwait(false);
+#endif
                 string responseContent = await httpResponse.Content.ReadAsStringAsync().ConfigureAwait(false);
 
                 var exception =