Description:
------------
As part of generating the body of our soap request we use the special three-argument form of createDocument that takes an exising SDO. This is misbehaving when the SDO that it is passed contains a null.

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

$xsd = <<<EOF
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" 
  targetNamespace="http://ConvertedStockQuote">
  <xs:element name="getQuote">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="ticker" type="xs:string" nillable="true"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

EOF;
file_put_contents('bug.xsd',$xsd);

$xmldas        = SDO_DAS_XML::create('bug.xsd');
$get_quote_sdo = $xmldas->createDataObject('http://ConvertedStockQuote','getQuote');
$get_quote_sdo ->ticker = 'IBM';
$xdoc2         = $xmldas->createDocument(null, 'getQuote', $get_quote_sdo);
$xmlstr        = $xmldas->saveString($xdoc2,2);
echo $xmlstr;

$get_quote_sdo = $xmldas->createDataObject('http://ConvertedStockQuote','getQuote');
$get_quote_sdo ->ticker = null;
$xdoc2         = $xmldas->createDocument(null, 'getQuote', $get_quote_sdo);
$xmlstr        = $xmldas->saveString($xdoc2,2);
echo $xmlstr;
?>

Expected result:
----------------
In the result of the second echo I want to see a null ticker like this:

  <ticker xmlns="http://ConvertedStockQuote" xsi:nil="true"/>


Actual result:
--------------
What I actually see has getQuote where I expect to see ticker

<getQuote xmlns="http://ConvertedStockQuote" xsi:nil="true"/>

and what's more I have a getQuote embeddded in a getQUote.