Introduce helpers for params

Improvements

• Add helper methods to easily get permitted params for the primary resource and its relationships:
• resource_params: helper to get the permitted params from params['data']['attributes'];
• relationship_params: helper to get ids from params['relationships'].

Fixes

• Work with a copy of ActionController's params in JSONAPI::Utils::Request#setup_request to avoid the bug reported in #21.

More details: #23