wp.api.models.Post.save method should not send date fields for drafts

[goldenapples]

When a new Post model is created through the REST API client, it will be hydrated complete with "date_gmt" and "date_modified_gmt" properties as a result of the changeset in https://core.trac.wordpress.org/changeset/40115

However, those properties are not filtered out before saving the model, and so the date_gmt field is posted as part of the save request. This causes inconsistencies with the way core leaves the post_date_gmt field as zeroes for unpublished posts.

In addition, if the "date" field is posted when saving the model, a "date_gmt" field will be created from it in the WP_REST_Posts_Controller::prepare_item_for_database method.

I'm not sure what the best way of addressing this is, but it seems like two changes are in order.

• Perhaps the API schema for a post could set a flag indicating whether the value returned for 'date_gmt' or 'date_modified_gmt' is a real value or a shimmed date that should not be saved, and, if that flag is set, the Backbone client would be responsible for unsetting that field before saving.

• In WP_REST_Posts_Controller::prepare_item_for_database, a draft post with a "date" field set shouldn't be passing a value in post_date_gmt along to wp_update_post - that's different behavior than saving in any other way and is unexpected. I could well be missing something, but it seems like that logic could be removed safely.

[danielbachhuber]

Bumping out of 5.0 because it's not necessary for Gutenberg.

[TimothyBlynJacobs]

I think this has now been fixed on the server side of things #39953.