Serialize OpenApiDate values properly to short date (#1102)

* Serialize OpenApiDate properly as Date

* Change test date value

Using value 01/01/0001 could be represented as 1/1/0001 in a machine using a different regional format, and thus cause the test to fail

src/Microsoft.OpenApi/Any/OpenApiPrimitive.cs

@@ -118,7 +118,7 @@ public void Write(IOpenApiWriter writer, OpenApiSpecVersion specVersion)
 
                 case PrimitiveType.Date:
                     var dateValue = (OpenApiDate)(IOpenApiPrimitive)this;
-                    writer.WriteValue(dateValue.Value);
+                    writer.WriteValue(dateValue.Value.ToShortDateString());
                     break;
 
                 case PrimitiveType.DateTime:

test/Microsoft.OpenApi.Tests/Models/OpenApiExampleTests.SerializeReferencedExampleAsV3JsonWithoutReferenceWorks_produceTerseOutput=False.verified.txt

@@ -21,6 +21,7 @@
           }
         ]
       }
-    ]
+    ],
+    "aDate": "12/12/2022"
   }
 }

test/Microsoft.OpenApi.Tests/Models/OpenApiExampleTests.SerializeReferencedExampleAsV3JsonWithoutReferenceWorks_produceTerseOutput=True.verified.txt

@@ -1 +1 @@
-{"value":{"versions":[{"status":"Status1","id":"v1","links":[{"href":"http://example.com/1","rel":"sampleRel1"}]},{"status":"Status2","id":"v2","links":[{"href":"http://example.com/2","rel":"sampleRel2"}]}]}}
+{"value":{"versions":[{"status":"Status1","id":"v1","links":[{"href":"http://example.com/1","rel":"sampleRel1"}]},{"status":"Status2","id":"v2","links":[{"href":"http://example.com/2","rel":"sampleRel2"}]}],"aDate":"12/12/2022"}}

test/Microsoft.OpenApi.Tests/Models/OpenApiExampleTests.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license. 
 
+using System;
 using System.Globalization;
 using System.IO;
 using System.Text;
@@ -95,7 +96,8 @@ public class OpenApiExampleTests
                             }
                         }
                     }
-                }
+                },
+                ["aDate"] = new OpenApiDate(DateTime.Parse("12/12/2022 00:00:00"))
             }
         };