Inputs inside disabled fieldset are submited on remote: true forms

[duleorlovic]

Steps to reproduce

Inside disabled fieldset, all controls are disabled mozilla.org and that works as intended for local: true forms (those fields are not send when form is submitted).
When we use local: false forms those input fields are send to server:

<%= form_with(model: post, local: false) do |form| %>
  <fieldset disabled>
    <%= form.text_field :title %>
  </fieldset>
  <%= form.submit %>
<% end %>

Expected behavior

It should not send title field.

Actual behavior

In above example, when form is remote (ajax), title field is sent even it is inside disabled fieldset.

System configuration

I notice this issue is on Rails 5.1.0 and later
On Rails 4.2.11 and 5.0.0 it does not happen.