Add digest config option for SignedCookiesJar

Two commits with small improvements:

* `config.action_dispatch.cookies_digest` config option
* `AS::MessageVerifier` now returns object preconfigured with `config.action_dispatch.cookies_serializer` && `config.action_dispatch.cookies_digest` values.

/cc @chancancode @tamird
Initially wanted to do it like this:

```
get :set_signed_cookie
cookies_sha1 = @controller.send :cookies
@request.env["action_dispatch.cookies_digest"] = 'SHA256'
get :set_signed_cookie
cookies_sha256 = @controller.send :cookies
```

and compare the two, but after first request verifier object gets cached.