Adding CookirJar #fetch method for hash-like access to cookies

So many times I have found myself doing something like this:

```ruby
class HomeController
  def index
    cookies.fetch(:some_key)
  end
end
```

Of course, I'm immediately informed that the fetch method does not exist on the `CookieJar`. I got tired of this because cookies for often are treated like a hash.

Let's give `cookies` a little promotion, shall we?