How to mock HttpContext.Current.Items with NUnit and Rhino Mocks

I'm using NUnit and RhinoMocks for unit testing on the (WebApi) project.

There is a method I'm trying to write test for, which is supposed to add an item to HttpContext.Current.Items.

public override void OnActionExecuting(HttpActionContext actionContext)
{
    HttpContext.Current.Items.Add("RequestGUID", Guid.NewGuid());
    base.OnActionExecuting(actionContext);
}

I have no idea how can I make HttpContext.Current.Items available to the method when ran from within a test method. How can I achieve this?

Also, how can I check if the item has been added (what kind of assertion can/should I use)