PUT course_details updates inconsistently (possibly requires start_date?)

When exercising the API:

PUT /api/contentstore/v1/course_details/<key>
(cms/djangoapps/contentstore/rest_api/v1/views/course_details.py /
openedx.core.djangoapps.models.course_details.CourseDetails.update_from_json)

The view documents partial bodies ("multiple details can be updated in a single request"), and every other field measured seems to honor one. self_paced does not: {"self_paced": true} (or "true", the string the MFE sends) answers HTTP 200 with the unchanged pacing echoed back, and a subsequent GET agrees.

Adding the course's own current start_date to the same body - {"self_paced": true, "start_date": "2040-01-01T00:00:00Z"} - makes the same request take effect.

Side effect: with the pacing change dropped, the accompanying certificates_display_behavior reset to EARLY_NO_INFO still happens, leaving an instructor-paced course with the self-paced display behavior.

Note: that this doesn't impact the MFE, which always sends the whole form, but is confusing behavior for any API consumer.

Note: a body carrying self_paced: true together with a start date in the past also leaves pacing unchanged with HTTP 200.

Start in cms/djangoapps/contentstore/rest_api/v1/views/course_details.py and openedx.core.djangoapps.models.course_details.CourseDetails.update_from_json. Reproduce a partial PUT containing self_paced, including the string value and a past start_date, then compare the response and a subsequent GET. Done means self_paced updates without requiring start_date and certificates_display_behavior is not incorrectly reset.