Array to string conversion in panels_cache_object->cache() warnings present on the page

Problem/Motivation

Warning: Array to string conversion in panels_cache_object->cache() (line 158 of /mnt/www/html/jnjnab4d1dev8/docroot/profiles/jjbos/modules/contrib/panels/includes/plugins.inc).

Steps to reproduce

1. Open the main application page.
2. Log in as admin.
3. Go to /admin/structure/pages.
4. Click on the 'Edit' link in a row of any existing panel page.
5. Click on the 'Content' link.
6. Click on the gear icon near one of the blocks.
7. Click on Caching->Change.
8. Click on the 'Simple Cache' radio button and click 'Next'.
9. Select '15 sec' in Lifetime and click the "Save' button.
10. Click on the 'Update and Save' button.
11. Go to /admin/config/development/performance.
12. Click on the 'Clear all caches' button.
13. Go to a page from step 4.

Proposed resolution

Here is a commit - https://git.drupalcode.org/project/panels/-/commit/13c4dc1492a74e3dd82c3...
introduced the issue ( 11 year ago, during migration from Drupal 6 to Drupal 7, possibly)

-    $this->head = str_replace($this->head, '', drupal_set_html_head());
+   $this->head = str_replace($this->head, '', drupal_add_html_head());

The first problem: drupal_set_html_head always returns a string, and str_replace makes sense.
drupal_add_html_head always returns an array, and the sense is not as clear as before.

Second problem in function restore:

  function restore() {
    if (!empty($this->head)) {
   -   drupal_set_html_head($this->head);
   +   drupal_add_html_head($this->head);

Before change it made sense, because inside drupal_set_html_head we did $stored_head .= $data ."\n";
After the change this line is useless, because drupal_add_html_head withour $key param will not add anything

Remaining tasks

First issue should be fixed by MR ( it is as much close as possible to initial behaviour, in my view)

As a solution for second problem we just might just remove the

    if (!empty($this->head)) {
      drupal_add_html_head($this->head);
    }

from restore function