Fix #529: Support serializing the Enum class object

[magro]

When an Enum.class instance is serialized (by ClassSerializer), this
causes the creation of the registered default serializer (EnumSerializer)
with this call chain:

ClassSerializer.write
 -> Kryo.writeClass
  -> DefaultClassResolver.writeClass
   -> Kryo.getDefaultSerializer
    -> ReflectionSerializerFactory.makeSerializer(kryo, EnumSerializer, Enum.class)

This is done so that Kryo.writeClass can write the class name or the id
of the serializer registration.

Until now, the EnumSerializer failed instantiation for a type with null
enum constants. This however is the case for Enum.class itself.
The creation of EnumSerializer for Enum.class is now allowed, under the
assumption that this happens in the context of the Enum.class serialization,
and that write/read on the created EnumSerializer instance will never
be called. This assumption should be safe, since there should never be an instance
of the abstract Enum class.

[pardom]

Is this fix in the snapshots repo yet?

[magro]

@ParDonut yes it should.

[magro]

@ParDonut It got also just released with 4.0.2