Add culture invariant writer.

Author: bdebaere

src/Microsoft.OpenApi/Extensions/OpenApiSerializableExtensions.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license. 
 
+using System.Globalization;
 using System.IO;
 using Microsoft.OpenApi.Exceptions;
 using Microsoft.OpenApi.Interfaces;
@@ -62,13 +63,14 @@ public static void Serialize<T>(
             }
 
             IOpenApiWriter writer;
+            var streamWriter = new FormattingStreamWriter(stream, CultureInfo.InvariantCulture);
             switch (format)
             {
                 case OpenApiFormat.Json:
-                    writer = new OpenApiJsonWriter(new StreamWriter(stream));
+                    writer = new OpenApiJsonWriter(streamWriter);
                     break;
                 case OpenApiFormat.Yaml:
-                    writer = new OpenApiYamlWriter(new StreamWriter(stream));
+                    writer = new OpenApiYamlWriter(streamWriter);
                     break;
                 default:
                     throw new OpenApiException(string.Format(SRResource.OpenApiFormatNotSupported, format));

src/Microsoft.OpenApi/Writers/FormattingStreamWriter.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Microsoft.OpenApi.Writers
+{
+    public class FormattingStreamWriter : StreamWriter
+    {
+        public FormattingStreamWriter(Stream stream, IFormatProvider formatProvider)
+            : base(stream)
+        {
+            this.FormatProvider = formatProvider;
+        }
+
+        public override IFormatProvider FormatProvider { get; }
+    }
+}