Fix wrong argument in MainController#respond_to_missing?

Fixes #3454

Author: mshibuya

app/controllers/rails_admin/main_controller.rb

@@ -49,7 +49,7 @@ def method_missing(name, *args, &block)
     end
 
     def respond_to_missing?(sym, include_private)
-      if RailsAdmin::Config::Actions.find(name.to_sym)
+      if RailsAdmin::Config::Actions.find(sym)
         true
       else
         super

spec/controllers/rails_admin/main_controller_spec.rb

@@ -190,6 +190,13 @@ def get(action, params)
     end
   end
 
+  describe '#respond_to_missing?' do
+    it 'returns the result based on existence of action' do
+      expect(controller.send(:respond_to_missing?, :index, false)).to be true
+      expect(controller.send(:respond_to_missing?, :invalid_action, false)).to be false
+    end
+  end
+
   describe '#get_collection' do
     let(:team) { FactoryBot.create :team }
     let!(:player) { FactoryBot.create :player, team: team }