Handle nested fields_for by adding indexes to record_name

In case of the form with nested fields_for, i. e.

<%= form_for :foos, url: root_path do |f| %>
    <% @foos.each do |foo| %>
        <%= f.fields_for 'foo[]', foo do |f2| %>
            <%= f2.text_field :id %>
            <% foo.bars.each do |bar| %>
                <%= f2.fields_for 'bar[]', bar do |b| %>
                    <%= b.text_field :id %>
                <% end %>
            <% end %>
        <% end %>
     <% end %>
    <%= f.submit %>
<% end %>

rails doesn't add index for 'foo' in the inner fields_for block, so field names
in the outer fields_for looks like "foos[foo][#{foo_index}][id]" and in the
inner "foos[foo[]][bar][#{bar_index}][id]". Submitting of such form leads to an
error like:
>ActionController::BadRequest (Invalid request parameters: expected Array
>(got Rack::QueryParser::Params) for param `foo'):

This commit adds indexes for the foos in the inner blocks, so field names
become "foos[foo][#{foo_index}][bar][#{bar_index}][id]" and submitting of such
form works fine as expected.

Fixes #15332

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