Add a test for serialization of expandable fields

src/StripeTests/Infrastructure/ExpandableSerializationTest.cs

@@ -0,0 +1,87 @@
+namespace StripeTests
+{
+    using Newtonsoft.Json;
+    using Stripe;
+    using Stripe.Infrastructure;
+    using Xunit;
+
+    public class ExpandableSerializationTest : BaseStripeTest
+    {
+        [Fact]
+        public void SerializeNotExpanded()
+        {
+            var obj = new TestTopLevelObject
+            {
+                NestedId = "id_not_expanded",
+                Nested = null,
+            };
+
+            var expected = "{\n  \"nested\": \"id_not_expanded\"\n}";
+            Assert.Equal(expected, obj.ToJson().Replace("\r\n", "\n"));
+        }
+
+        [Fact]
+        public void SerializeExpanded()
+        {
+            var nested = new TestNestedObject
+            {
+                Id = "id_expanded",
+                Bar = 42,
+            };
+            var obj = new TestTopLevelObject
+            {
+                NestedId = nested.Id,
+                Nested = nested,
+            };
+
+            var expected =
+                "{\n  \"nested\": {\n    \"id\": \"id_expanded\",\n    \"bar\": 42\n  }\n}";
+            Assert.Equal(expected, obj.ToJson().Replace("\r\n", "\n"));
+        }
+
+        [Fact]
+        public void SerializeNull()
+        {
+            var obj = new TestTopLevelObject
+            {
+                NestedId = null,
+                Nested = null,
+            };
+
+            var expected = "{\n  \"nested\": null\n}";
+            Assert.Equal(expected, obj.ToJson().Replace("\r\n", "\n"));
+        }
+
+        private class TestNestedObject : StripeEntity, IHasId
+        {
+            [JsonProperty("id")]
+            public string Id { get; set; }
+
+            [JsonProperty("bar")]
+            public int Bar { get; set; }
+        }
+
+        private class TestTopLevelObject : StripeEntity
+        {
+            [JsonIgnore]
+            public string NestedId { get; set; }
+
+            [JsonIgnore]
+            public TestNestedObject Nested { get; set; }
+
+            [JsonProperty("nested")]
+            internal object InternalNested
+            {
+                get
+                {
+                    return this.Nested ?? (object)this.NestedId;
+                }
+
+                set
+                {
+                    StringOrObject<TestNestedObject>.Map(value, s => this.NestedId = s, o => this.Nested = o);
+                }
+            }
+        }
+    }
+}