Adding CookieJar #fetch methods 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 `CookieJar#fetch` method doesn't exist. I got tired of this because cookies are so often treated like `Hash`es.

Let's give `cookies` a little promotion, shall we?