Update WebApiFactory.cs (#153)

This could fix the ImplicitGrantAuth for http paths without breaking anything. ´TrimEnd` is there for when there is no path (the builder will add a `/` which will make the callback url invalid otherwise.

SpotifyAPI/Web/Auth/WebApiFactory.cs

@@ -35,7 +35,7 @@ namespace SpotifyAPI.Web.Auth
         {
             var authentication = new ImplicitGrantAuth
             {
-                RedirectUri = $"{_redirectUrl}:{_listeningPort}",
+                RedirectUri = new UriBuilder(_redirectUrl) { Port = _listeningPort }.Uri.OriginalString.TrimEnd('/'),
                 ClientId = _clientId,
                 Scope = _scope,
                 State = _xss