Default values in meta boxes are never saved for new posts

[BruceOnTheGo]

Issue Overview

When a plugin adds a meta box to the editor and the meta box has some default values the default values are not saved when creating a new post. The values are only saved if the default values are changed. This is not the normal behaviour with the classic editor. The default values are always saved.

Using master branch.

Steps to Reproduce (for bugs)

1. Use the following plugin code
2. Click on new page
3. Add a title
4. Publish the post

<?php
/*
Plugin Name: Example for meta box with gutenberg
Author: OnTheGoSystems
Version: 1.0
*/

global $pagenow;

if ( in_array( $pagenow, array( 'post.php', 'post-new.php', 'edit.php' ), true ) ) {
	add_action( 'admin_head', 'gutenberg_new_post_add_meta_box' );
}

function gutenberg_new_post_add_meta_box() {
	add_meta_box( 'gutenberg_div', 'Options', 'gutenberg_meta_box', null, 'side', 'high' );
}

function gutenberg_meta_box() {
	?>
		<select name="gutenberg_select_test">
			<option value="1">Value 1</option>
			<option value="2">Value 2</option>
			<option value="3">Value 3</option>
		</select>

	<?php
}

add_action( 'save_post', 'gutenberg_save_post_example' );

function gutenberg_save_post_example( $post_id ) {

	if ( isset( $_POST['gutenberg_select_test'] ) ) {
		update_post_meta( $post_id, 'gutenberg_select_test', $_POST['gutenberg_select_test'] );
	}
}

Expected Behavior

The 'save_post' action should be triggered and the update_post_meta function is called. This is what happens with the classic editor

Current Behavior

The 'save_post' action is not triggered and the post meta is not updated.

Possible Solution

I think all meta boxes should be considered as 'dirty' for new posts and all the meta box data should be submitted when saving the initial post.

[BruceOnTheGo]

Possibly related to #3730