Register _classifai_error meta as single

[johnwatkins0]

Description of the Change

This is a one-line change intended to resolve #105. The description of the issue this addresses can be found there. Essentially, JS is expecting a WP meta value returned from the REST API to be a string that can be parsed as JSON, and the REST API is returning an array. To fix this, we update the configuration passed to the WP register_meta function to tell to tell WP to expect a single value rather than an array.

See: https://codex.wordpress.org/Function_Reference/register_meta

Alternate Designs

If there is any likelihood multiple values of _classifai_error might be intentionally associated with a given post, then this fix might instead involve additional checks at the JS level to validate whether the error notice should display. Currently, however, _classifai_error is assumed to be a single value elsewhere in the plugin, including in SavePostHandler.php where the value is updated, retrieved, and deleted.

Benefits

"Post updated" notices display as expected. Elimination of JS error reduces risk of other unknown side effects.

Verification Process

With all features of the plugin correctly configured, edit a post with the JS console open. When saving the post, confirm the JS error screenshotted in #105 doesn't show, and that the relevant "Post updated" notice shows.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

The file I touched is not currently covered by tests (beyond testing the class works as a singleton) so I didn't add any tests, but I'd be happy to add an integration test confirming the meta value in REST responses if that would be considered beneficial.

Applicable Issues

#105