protected override async Task RemoveUserTokenAsync(Model.IdentityUserToken <string> token)
        {
            var doc = await Context.Client.DeleteDocumentAsync(token.SelfLink,
                                                               Context.RequestOptions);

            Context.SetSessionTokenIfEmpty(doc.SessionToken);
        }
        protected override async Task AddUserTokenAsync(Model.IdentityUserToken <string> token)
        {
            var doc = await Context.Client.CreateDocumentAsync(Context.IdentityDocumentCollection.DocumentsLink, token
                                                               , Context.RequestOptions, true);

            Context.SetSessionTokenIfEmpty(doc.SessionToken);
            token = JsonHelpers.CreateObject <Model.IdentityUserToken <string> >(doc);
        }