make attribute MsgBoxRequestID in MsgBoxCloseRequest optional

src/main/resources/wsdl/OSCI2_02.xsd

@@ -153,7 +153,7 @@
     <xs:sequence minOccurs="0">
       <xs:element name="LastMsgReceived" type="wsa:AttributedURIType" maxOccurs="unbounded"/>
     </xs:sequence>
-    <xs:attribute name="MsgBoxRequestID" type="xs:anyURI" use="required"/>
+    <xs:attribute name="MsgBoxRequestID" type="xs:anyURI" use="optional"/>
   </xs:complexType>
   <!--Types and Elements for Receipt- and Notification Handling-->
   <xs:attribute name="qualTSPForReceipt" type="xs:boolean" default="false"/>