Properly dispose http server

When using the AuthorizationCodeAuth flow, the HTTP server should be stopped properly like in ImplicitGrantAuth

SpotifyAPI/Web/Auth/AutorizationCodeAuth.cs

@@ -111,6 +111,7 @@ namespace SpotifyAPI.Web.Auth
         /// </summary>
         public void StopHttpServer()
         {
+            _httpServer.Dispose();
             _httpServer = null;
         }
 
@@ -159,4 +160,4 @@ namespace SpotifyAPI.Web.Auth
         public String State { get; set; }
         public String Error { get; set; }
     }
-}
+}