[Java] Fix serializing issues of RaySerializer (#4887)

* Fix

* Address comment.

java/test/src/main/java/org/ray/api/test/RaySerializerTest.java

@@ -0,0 +1,23 @@
+package org.ray.api.test;
+
+import org.ray.api.RayPyActor;
+import org.ray.api.id.UniqueId;
+import org.ray.runtime.RayPyActorImpl;
+import org.ray.runtime.util.Serializer;
+import org.testng.Assert;
+import org.testng.annotations.Test;
+
+public class RaySerializerTest {
+
+  @Test
+  public void testSerializePyActor() {
+    final UniqueId pyActorId = UniqueId.randomId();
+    RayPyActor pyActor = new RayPyActorImpl(pyActorId, "test", "RaySerializerTest");
+    byte[] bytes = Serializer.encode(pyActor);
+    RayPyActor result = Serializer.decode(bytes);
+    Assert.assertEquals(result.getId(), pyActorId);
+    Assert.assertEquals(result.getModuleName(), "test");
+    Assert.assertEquals(result.getClassName(), "RaySerializerTest");
+  }
+
+}