Fix enrichment serialization error (#135)

* add JsonIgnore annotation to enrichment

* add unit tests to cover events with enrichment writing to disk

Author: wenxi-zeng

Analytics-CSharp/Segment/Analytics/Types.cs

@@ -1,6 +1,12 @@
 using global::System;
 using Segment.Serialization;
 
+#if NETSTANDARD2_0
+using System.Text.Json.Serialization;
+#else
+using Newtonsoft.Json;
+#endif
+
 namespace Segment.Analytics
 {
     public class DestinationMetadata
@@ -18,6 +24,7 @@ public abstract class RawEvent
         public virtual string UserId { get; set; }
         public virtual string Timestamp { get; set; }
 
+        [JsonIgnore]
         public Func<RawEvent, RawEvent> Enrichment { get; set; }
 
         // JSON types

Tests/Utilities/StorageTest.cs

@@ -254,6 +254,30 @@ public async Task TestStoreEvent()
             Assert.Null(exception);
         }
 
+        [Fact]
+        public async Task TestStoreEventWithEnrichment()
+        {
+            TrackEvent trackEvent = new TrackEvent("clicked", new JsonObject { ["foo"] = "bar" });
+            trackEvent.Enrichment = @event =>
+            {
+                return @event;
+            };
+            string payloadWithEnrichment = JsonUtility.ToJson(trackEvent);
+            await _storage.Write(StorageConstants.Events, payloadWithEnrichment);
+            await _storage.Rollover();
+
+            string path = _dir + Path.DirectorySeparatorChar + _writeKey + "-0.json";
+            string actual = File.ReadAllText(path);
+            Exception exception = Record.Exception(() =>
+            {
+                JsonUtility.FromJson<JsonObject>(actual);
+            });
+
+            Assert.True(File.Exists(path));
+            Assert.Contains(_payload, actual);
+            Assert.Null(exception);
+        }
+
         [Fact]
         public async Task TestRead()
         {