Fix the value of submit buttons being lost on submisison

Fixes #3513

Author: mshibuya

spec/integration/actions/edit_spec.rb

@@ -766,6 +766,22 @@ class HelpTest < Tableless
     end
   end
 
+  context 'with a submit button with custom value', js: true do
+    before do
+      @player = FactoryBot.create :player
+
+      visit edit_path(model_name: 'player', id: @player.id)
+
+      execute_script %{$('.form-actions [name="_save"]').attr('name', 'player[name]').attr('value', 'Jackie Robinson')}
+      find_button('Save').trigger('click')
+    end
+
+    it 'submits the value' do
+      @player.reload
+      expect(@player.name).to eq('Jackie Robinson')
+    end
+  end
+
   context 'with missing object' do
     before do
       put edit_path(model_name: 'player', id: 1), params: {player: {name: 'Jackie Robinson', number: 42, position: 'Second baseman'}}

src/rails_admin/ui.js

@@ -92,7 +92,7 @@ import I18n from "./i18n";
           $("<input />")
             .attr("type", "hidden")
             .attr("name", $(this).attr("name"))
-            .attr("value", true)
+            .attr("value", $(this).attr("value"))
         );
         if ($(this).is("[formnovalidate]")) {
           form.attr("novalidate", true);