Description:
------------
SDO doesn't enforce use="required" for attributes.  It allows for the absence of the attribute which violates the schema.

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:element name="collection" type="xsd:token"/>
      <xsd:attribute name="kind" type="xsd:string" fixed="collectionInfo" use="required"/>
    </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>



Expected result:
----------------
Throw an exception when inserting an object that does not have the required property set.

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