public void Id_is_mapped_back_to_url() {
var sut = new UrlMapping();
var id = sut.Id_for_Url("a");
var url = sut.Url_for_id(id);
Assert.That(url, Is.EqualTo("a"));
}
public void Url_for_Url_returns_an_id() {
var sut = new UrlMapping();
var id = sut.Id_for_Url("a");
Assert.That(id.Length, Is.EqualTo(6));
}