public static Task DeleteIllustBookmarkAsync(this PixivClient client, IllustID id)
 {
     client.RequireAuth();
     return(client.PostAsync("/v1/illust/bookmark/delete", new HttpForm {
         { "illust_id", id }
     }));
 }
 // TODO support tag
 public static Task AddIllustBookmarkAsync(this PixivClient client, IllustID id, Restrict restrict = Restrict.Public)
 {
     client.RequireAuth();
     return(client.PostAsync("/v2/illust/bookmark/add", new HttpForm {
         { "illust_id", id }, { "restrict", restrict.GetSymbol() }
     }));
 }