Fix custom cookies getting overwritten

[Guacam-Ole]

To allow providing a custom Cookie like mentioned in the Authentication-part of the Readme:

using YoutubeExplode;

// Perform authentication and extract cookies
var cookies = ...;

// Cookie collection must be of type IReadOnlyList<System.Net.Cookie>
var youtube = new YoutubeClient(cookies);

That custom cookie must always have priority over the default/fallback cookie for this to work. I changed the order so to meet this requirement

Closes #909