Add System.Text.Json ExtensionData attribute tests (#1312)

* Add System.Text.Json extension attribute tests

* Fix test

Co-authored-by: Rico Suter <mail@rsuter.com>

src/NJsonSchema.Tests/Generation/SystemTextJson/SystemTextJsonExtensionDataGenerationTests.cs

@@ -0,0 +1,38 @@
+#if !NET46 && !NET452
+
+using System.Collections.Generic;
+using System.Text.Json.Serialization;
+using NJsonSchema.Generation;
+using Xunit;
+
+namespace NJsonSchema.Tests.Generation.SystemTextJson
+{
+    public class SystemTextJsonExtensionDataGenerationTests
+    {
+        public class ClassWithExtensionData
+        {
+            public string Foo { get; set; }
+
+            [JsonExtensionData]
+            public IDictionary<string, object> ExtensionData { get; set; }
+        }
+
+        [Fact]
+        public void SystemTextJson_When_class_has_property_with_JsonExtensionDataAttribute_on_property_then_AdditionalProperties_schema_is_set()
+        {
+            //// Act
+            var schema = JsonSchema.FromType<ClassWithExtensionData>(new JsonSchemaGeneratorSettings
+            {
+                SchemaType = SchemaType.OpenApi3,
+                SerializerOptions = new System.Text.Json.JsonSerializerOptions()
+            });
+
+            //// Assert
+            Assert.Equal(1, schema.ActualProperties.Count);
+            Assert.True(schema.AllowAdditionalProperties);
+            Assert.True(schema.AdditionalPropertiesSchema.ActualSchema.IsAnyType);
+        }
+    }
+}
+
+#endif

src/NJsonSchema/Generation/SystemTextJsonUtilities.cs

@@ -56,8 +56,9 @@ protected override JsonProperty CreateProperty(MemberInfo member, MemberSerializ
                 var attributes = member.GetCustomAttributes(true);
 
                 var property = base.CreateProperty(member, memberSerialization);
-                property.Ignored = attributes
-                    .FirstAssignableToTypeNameOrDefault("System.Text.Json.Serialization.JsonIgnoreAttribute", TypeNameStyle.FullName) != null;
+                property.Ignored = 
+                    attributes.FirstAssignableToTypeNameOrDefault("System.Text.Json.Serialization.JsonIgnoreAttribute", TypeNameStyle.FullName) != null ||
+                    attributes.FirstAssignableToTypeNameOrDefault("System.Text.Json.Serialization.JsonExtensionDataAttribute", TypeNameStyle.FullName) != null;
 
                 if (_serializerOptions.PropertyNamingPolicy != null)
                 {