updates for v2 API and ping controller

- [x] wrapped single-object responses with a root-node
- [x] make single-object response root-node optional
- [x] read http param for root-node name
- [x] ping action
- [x] other ping controller actions for v2
- [x] tests

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will be included in a separate PR that contains collection responses
- [ ] wrapped collection responses with a root-node
- [ ] collection response metadata

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original PR is #3438. it was closed because the `engine` branch has now
been merged into `master`.