enum value should be serializable and should also keep referential integrity

[JurajBurian]

// let say that "enum" WeekDay with value Mon is defined
// then this test should be ok:
  val out = new ByteArrayOutputStream()
  new ObjectOutputStream(out).writeObject(Mon);
  out.close();
  val monday = new ObjectInputStream(new ByteArrayInputStream(out.toByteArray)).readObject()
  assert(monday == Mon)

[joroKr21]

See #1271 for an example