Generated properties/routes names don't take into account JSONAPI::Resources key/route formatters

If the following `JSONAPI::Resources` configuration is used:
```
JSONAPI.configure do |config|
  config.json_key_format = :camelized_key
...
```
it is expected to see camelized properties names in generated result, e.g. `createdAt` instead of `created_at`.

Probably `JSONAPI.configuration.key_formatter / JSONAPI.configuration.route_formatter` should be used.
