Prevent `Padrino::Cache#registered` from overriding cache and caching

Prevent `Padrino::Cache#registered` from overriding both `cache` and `caching` settings if set before the call to `register Padrino::Cache`.

```
  # app.rb
  enable :caching
  register Padrino::Cache

  # app.caching #> false
```

That's because of the explicit [disable](https://github.com/padrino/padrino-framework/blob/master/padrino-cache/lib/padrino-cache.rb#L102) we set by default.

This issue will also arise when doing a global config in `config/apps.rb` and it's trickier to track over there.