Unit testing WebApi 2 ApiController with ExecuteAsync overriden

I have ApiController with ExecuteAsync method overriden like this (I am using RavenDB and here I create its session):

public override async Task<HttpResponseMessage> ExecuteAsync(
        HttpControllerContext controllerContext,
        CancellationToken cancellationToken)
{
    using (Session = Store.OpenAsyncSession())
    {
        var result = await base.ExecuteAsync(controllerContext, cancellationToken);
        await Session.SaveChangesAsync();
        return result;
    }
}

I want to write automated Tests for it using xUnit or Microsoft Test Framework and at the moment I'm stuck with calling controllers action via ExecuteAsync.

Should I construct ControllerContext in any particular way to have that done or am I missing something?

Simply creating controller object and calling actions throws NullReferenceException for Session object, which is obviously not created as the ExecuteAsync has not been called.