EventJsonTest: Add test for custom event serialization

Change-Id: I7e45f77aa980c03896e3891c2d48680e31d2e0a6
(cherry picked from commit 03707279ebf5bee6fb795b3d99be283310b33bf3)

javatests/com/google/gerrit/server/events/EventJsonTest.java

diff --git a/javatests/com/google/gerrit/server/events/EventJsonTest.java b/javatests/com/google/gerrit/server/events/EventJsonTest.java
index 6163ea7..9ca60ca 100644
--- a/javatests/com/google/gerrit/server/events/EventJsonTest.java
+++ b/javatests/com/google/gerrit/server/events/EventJsonTest.java
@@ -52,12 +52,32 @@
 
   private final Gson gson = new EventGsonProvider().get();
 
+  static class CustomEvent extends Event {
+    static String TYPE = "custom-type";
+
+    public String customField;
+
+    protected CustomEvent() {
+      super(TYPE);
+    }
+  }
+
   @Before
   public void setTimeForTesting() {
     TestTimeUtil.resetWithClockStep(1, TimeUnit.SECONDS);
   }
 
   @Test
+  public void customEvent() {
+    CustomEvent event = new CustomEvent();
+    event.customField = "customValue";
+    String json = gson.toJson(event);
+    CustomEvent resullt = gson.fromJson(json, CustomEvent.class);
+    assertThat(resullt.type).isEqualTo(CustomEvent.TYPE);
+    assertThat(resullt.customField).isEqualTo(event.customField);
+  }
+
+  @Test
   public void refUpdatedEvent() {
     RefUpdatedEvent event = new RefUpdatedEvent();