title: Microsoft.Activities.UnitTesting not cater Workflow Service with DefinitionIdentity
body: <p>I am using WF4.5 and Microsoft.Activities.UnitTesting. I defined DefinitionIdentity for my workflow service. When WorkflowServiceTestHost.Open is called, it will give the following exception:</p>
  
  <blockquote>
    <p>System.NotSupportedException: The persistence provider implementation
    of InstanceStore doesn't support the command named
    {urn:schemas-microsoft-com:System.Activities.Persistence/command}CreateWorkflowOwnerWithIdentity.
    Either choose a different provider, or ensure that this persistence
    command isn't attempted.</p>
  </blockquote>
  
  <p>It seems because Microsoft.Activities.UnitTesting.Persistence.MemoryStore didn't cater CreateWorkflowOwnerWithIdentityCommand. Anyone implemented it? Thanks.</p>