Description:
------------
I am able set more than one element in a choice when only one element should be allowed.

Reproduce code:
---------------
<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

<xsd:element name="request" type="requestType"/>

<xsd:complexType name="requestType" abstract="true"/>

<xsd:complexType name="collectionInfo">
  <xsd:complexContent>
    <xsd:extension base="requestType">
      <xsd:choice>
        <xsd:element name="collection" type="xsd:token"/>
        <xsd:element name="collection2" type="xsd:token"/>
      </xsd:choice>
      <xsd:attribute name="kind" type="xsd:string" fixed="collectionInfo"/>
    </xsd:extension>
  </xsd:complexContent>
</xsd:complexType>

<xsd:element name="request-list">
  <xsd:complexType>
     <xsd:sequence>
        <xsd:element ref="request" minOccurs="0" maxOccurs="unbounded"/>
     </xsd:sequence>
  </xsd:complexType>
</xsd:element>

</xsd:schema>

<?php

try {
	$xmldas = SDO_DAS_XML::create("request.xsd");
	try {
		$doc = $xmldas->createDocument('', 'request-list');
		$rdo = $doc->getRootDataObject();
		$request = $xmldas->createDataObject('', 'collectionInfo');
		$request->collection = 'foo bar';
		$request->collection2 = 'foo2 bar2';
		$request->kind = 'collectionInfo';
		$rdo->request->insert($request);
		print($xmldas->saveString($doc));
	} catch (SDO_Exception $e) {
		print($e);
	}
} catch (SDO_Exception $e) {
	print("Problem creating an XML document: " . $e->getMessage());
}

?>

Expected result:
----------------
Throw a choice exception stating that only one element can be set in a choice.

Actual result:
--------------
<?xml version="1.0" encoding="UTF-8"?>
<request-list xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><request xsi:type="collectionInfo" kind="collectionInfo"><collection>foo bar</collection><collection2>foo2 bar2</collection2></request></request-list>