ServiceTaskHandler ignores parameter interfaceImplementationRef of workItem

In bpmn2 process file, can be specified interface in way like:
<bpmn2:interface id="Interface_2" implementationRef="java.util.Date" name="java.util.Date">
<operation>
...
</operation>
...
</bpmn2:interface>

Currently, ServiceTaskHandler was assuming, that in name of interface, must be stored full qualified name of class, which will be instantiated.