[FIX] web: Settings headers should not be dirty

When a user changes a header setting, the record should not be marked as
dirty. If the user clicks on a button, the save dialog should not be
shown.

Next commit will bring a functional test and "steps to reproduce" of
this issue in website.

task-3265100

Part-of: odoo/odoo#125550
Co-authored-by: "Guillaume (gdi)" <gdi@odoo.com>