Unprocessable Content 422: The attribute metatag does not exist on the site--drupal resource type..

Problem/Motivation

When a site has a field that a site manager does not, an error is thrown when trying to send it:

Unprocessable Content 422: The attribute metatag does not exist on the site--drupal resource type..

Steps to reproduce

Enable site module on a site that has metatag.
Configure site.module to send to a site manager instance that does not have metatag installed.
Note error.

Proposed resolution

This is JSON API. Somehow, we are going to have to intercept the entity validation here.

Remaining tasks

Allow site entities to ignore fields they don't have.

User interface changes

None.

API changes

None.

Data model changes

None.