Description:
------------
I assign nulls to properties of an SDO then clone it. The nulls do not get into the cloned object. 

It does not matter by the way whether or not I have nillable="true" in the schema. 

Reproduce code:
---------------
<?php

$xsd = <<<EOF
<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
        targetNamespace="PersonNamespace"
        xmlns:AuthorNS="PersonNamespace">
  <complexType name="personType">
    <sequence>
      <element name="name" type="string"/>
      <element name="dob" type="string"/>
      <element name="pob" type="string"/>
    </sequence>
  </complexType>
</schema>
EOF;

file_put_contents('person.xsd',$xsd);

$xmldas = SDO_DAS_XML::create('person.xsd');

$person = $xmldas->createDataObject('PersonNamespace','personType');

$person->name = "William Shakespeare'";
$person->dob = null;
$person->pob = null;

$person2 = clone($person);
print($person);
echo "\n";
print($person2);

?>

Expected result:
----------------
I want to see nulls in dob abd pob for $person2. 

Actual result:
--------------
object(SDO_DataObject)#3 (3) {name=>"William Shakespeare'"; dob=>NULL; pob=>NULL}
object(SDO_DataObject)#4 (3) {name=>"William Shakespeare'"; dob; pob}