Consider integration tests for your WP REST API endpoints

Hi!

On behalf of [Pantheon](https://pantheon.io/), I'm [writing a tutorial](https://paper.dropbox.com/doc/You-absolutely-need-test-coverage-for-your-WP-REST-API-project-MKvKy9DzoTQ2HYhbyWBxM) on why integration tests are critical for your WP REST API endpoints, and how you can go about writing them.

Given your use of `register_rest_field()`, I think you could benefit from reading and applying the tutorial. And, because the tutorial isn't yet published, I'd very much appreciate any constructive criticism you have, so that we can make the tutorial an evergreen resource for the community.

Thanks in advance for your time. Happy to take any questions, comments, etc. you might have in this Github issue, or on the document itself (you'll need to be signed into Dropbox).

Read the linked integration-testing tutorial and inspect where this plugin uses register_rest_field(). The issue does not name files, endpoints, tests, or a concrete completion condition, so the required test scope would need to be clarified before work can begin.

Written by the indexing model from the issue text.