Track url added by #add_url

I found urls added by `#add_url` (manually or at the beginning of `#takeover`) are not tracked by `url_tracker`

This commit adds `url_tracker.visit` to `#add_url`

My question is, what is after this change the difference between `#add_url` and the private method `#enqueue`?

I have a second question regarding `#enqueue`.
```ruby
def(enqueue url_to_visit, current_page, queue)
```

why does `#enqueue` require a queue as attribute? Would it not be possible to use `@internal_queue`?