Declare generic type parameter as having a destructor?
public class EntityContextFactory<T>
    where T: class, IDisposable, IObjectContextAdapter, new()

T context = HttpContext.Current.Items[objectContextKey] as T;
            if (context != null)
            {
                context.Dispose();
                GC.SuppressFinalize(context);
                HttpContext.Current.Items.Remove(objectContextKey);
            }