Make flags field readonly

src/csharp/Grpc.Core/WriteOptions.cs

@@ -1,6 +1,6 @@
 #region Copyright notice and license
 
-// Copyright 2015, Google Inc.
+// Copyright 2015-2016, Google Inc.
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
@@ -64,7 +64,7 @@ namespace Grpc.Core
         /// </summary>
         public static readonly WriteOptions Default = new WriteOptions();
             
-        private WriteFlags flags;
+        private readonly WriteFlags flags;
 
         /// <summary>
         /// Initializes a new instance of <c>WriteOptions</c> class.