Updating a page transitions content to the new editor

Issue Summary

Updating the title of a page with legacy editor content using the REST API results in the the content switching to the new editor regardless of whether it can be safely converted or not.

Steps to Reproduce

1. Create a page with legacy editor content.
2. Modify the page to include content that can not be converted to the new editor format. For example, nested expands.
3. Update the title and content of the page using PUT /wiki/rest/api/content/{id}. Either `storage` or `editor2` content can be provided here.
4. Reload the page.
5. Use Property Inspector to view the version of the page. The version of the editor content is still "v1".
6. Update the title of the page using PUT /wiki/rest/api/content/{id}. Don't pass any new content within the `body` part of the request.
7. Reload the page.
8. Use Property Inspector to view the version of the page.

Expected Results

At step 8, the version of the editor content is still "v1" and renders as it did before.

Actual Results

At step 8, the version of the editor content has change to "v2" and does not render properly due to containing content that can not be safely converted to the new editor.

Workaround

None