Make the status helper accept status messages

Rack provides the `Rack::Utils.status_code` method which allows to retrieve a
response status code by its "symbolified" message: e.g. you can call
`status_code(:ok)` and it returns `200`. It's great also because if you pass
`'200'` or `200` to it, it still returns `200`.

By adding a call to `status_code` to the `status` method, nothing has been
removed or modified for the API. Now, however, you can call `status(:ok)` and it
will work as well.