How to test api controller that use an extension method as authorization check

I'm using ThinkTecture's resource based authorization in my WebApi.

I'm trying to test one of my controller that I need to check the access inside the function. But now, I can't test the function anymore since, I can't mock an extension method and since it's a nuget method, I can't modify the class to inject another value.

public async Task<IHttpActionResult> Get(int id)
{
    if (!(await Request.CheckAccessAsync(ChinookResources.AlbumActions.View, 
                                        ChinookResources.Album,
                                        id.ToString())))
    {
        return this.AccessDenied();
    }

    return Ok();
}

Source code of the ThinkTecture project is here.

Thank you for your help