Apply suggestions from code review

Author: baywet

src/Microsoft.OpenApi/Models/OpenApiMediaType.cs

@@ -111,8 +111,7 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
             else if (version < OpenApiSpecVersion.OpenApi3_2 && ItemSchema != null)
             {
                 // For v3.1 and earlier, serialize as x-oai-itemSchema extension
-                writer.WritePropertyName(OpenApiConstants.ExtensionFieldNamePrefix + "oai-" + OpenApiConstants.ItemSchema);
-                callback(writer, ItemSchema);
+                writer.WriteOptionalObject(OpenApiConstants.ExtensionFieldNamePrefix + "oai-" + OpenApiConstants.ItemSchema, ItemSchema, callback);
             }
 
             // example

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

@@ -478,9 +478,7 @@ public async Task SerializeMediaTypeWithItemSchemaAsV32JsonWorks()
             var actual = await mediaType.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_2);
 
             // Assert
-            actual = actual.MakeLineBreaksEnvironmentNeutral();
-            expected = expected.MakeLineBreaksEnvironmentNeutral();
-            Assert.Equal(expected, actual);
+            Assert.True(JsonNode.DeepEquals(JsonNode.Parse(actual), JsonNode.Parse(expected)));
         }
 
         [Fact]
@@ -517,9 +515,7 @@ public async Task SerializeMediaTypeWithItemSchemaAsV31JsonWorks()
             var actual = await mediaType.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_1);
 
             // Assert
-            actual = actual.MakeLineBreaksEnvironmentNeutral();
-            expected = expected.MakeLineBreaksEnvironmentNeutral();
-            Assert.Equal(expected, actual);
+            Assert.True(JsonNode.DeepEquals(JsonNode.Parse(actual), JsonNode.Parse(expected)));
         }
 
         [Fact]
@@ -556,9 +552,7 @@ public async Task SerializeMediaTypeWithItemSchemaAsV3JsonWorks()
             var actual = await mediaType.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0);
 
             // Assert
-            actual = actual.MakeLineBreaksEnvironmentNeutral();
-            expected = expected.MakeLineBreaksEnvironmentNeutral();
-            Assert.Equal(expected, actual);
+            Assert.True(JsonNode.DeepEquals(JsonNode.Parse(actual), JsonNode.Parse(expected)));
         }
 
         [Fact]