public static async Task<User> IsUserOnList(this ITwitterSession session, long listId, string slug="",
long userId = 0, string screenName = "", string ownerScreenName = "", long ownerId = 0)
{
var parameters = new TwitterParametersCollection();
parameters.Create(user_id:userId,screen_name:screenName, list_id: listId, slug: slug, owner_id: ownerId, owner_screen_name: ownerScreenName, skip_status:true,include_entities:true);
return await session.GetAsync(TwitterApi.Resolve("/1.1/lists/members/show.json"), parameters)
.ContinueWith(c => c.MapToSingle<User>());
}