make sync unary call behave more like async unary

src/csharp/Grpc.Core/Internal/AsyncCall.cs

@@ -80,8 +80,6 @@ namespace Grpc.Core.Internal
             using (profiler.NewScope("AsyncCall.UnaryCall"))
             using (CompletionQueueSafeHandle cq = CompletionQueueSafeHandle.CreateSync())
             {
-                byte[] payload = UnsafeSerialize(msg);
-
                 bool callStartedOk = false;
                 try
                 {
@@ -97,6 +95,8 @@ namespace Grpc.Core.Internal
                         readingDone = true;
                     }
 
+                    byte[] payload = UnsafeSerialize(msg);
+
                     using (var metadataArray = MetadataArraySafeHandle.Create(details.Options.Headers))
                     {
                         var ctx = details.Channel.Environment.BatchContextPool.Lease();