Fix the way `detect_common_display_methods` finds label & value methods

It was previously looking at an instance of the `collection` instead of the
`collection_classes`. Since the grouped collection input supplies a list
of collection classes that differ from the regular collection, it was
incorrectly looking for labelling and value methods on the option
instances, rather than the optgroup instances.

lib/simple_form/inputs/collection_input.rb

@@ -63,6 +63,7 @@ module SimpleForm
       end
 
       def detect_common_display_methods(collection_classes = detect_collection_classes)
+        return {} if collection_classes == []
         collection_translated = translate_collection if collection_classes == [Symbol]
 
         if collection_translated || collection_classes.include?(Array)
@@ -70,10 +71,10 @@ module SimpleForm
         elsif collection_includes_basic_objects?(collection_classes)
           { label: :to_s, value: :to_s }
         else
-          sample = collection.first || collection.last
+          sample = collection_classes.first
 
-          { label: SimpleForm.collection_label_methods.find { |m| sample.respond_to?(m) },
-            value: SimpleForm.collection_value_methods.find { |m| sample.respond_to?(m) } }
+          { label: SimpleForm.collection_label_methods.find { |m| sample.instance_methods.include?(m) },
+            value: SimpleForm.collection_value_methods.find { |m| sample.instance_methods.include?(m) } }
         end
       end
 

test/inputs/grouped_collection_select_input_test.rb

@@ -79,6 +79,24 @@ class GroupedCollectionSelectInputTest < ActionView::TestCase
     end
   end
 
+  test 'grouped collection finds default label methods on the group objects' do
+    option_list = ['Jose', 'Carlos']
+
+    GroupedClass = Struct.new(:to_label, :options)
+    group = GroupedClass.new("Authors", option_list)
+
+    with_input_for @user, :tag_ids, :grouped_select,
+      collection: [group],
+      group_method: :options
+
+    assert_select 'select.grouped_select#user_tag_ids' do
+      assert_select 'optgroup[label=Authors]' do
+        assert_select 'option', 'Jose'
+        assert_select 'option', 'Carlos'
+      end
+    end
+  end
+
   test 'grouped collection accepts label and value methods options' do
     with_input_for @user, :tag_ids, :grouped_select,
       collection: { 'Authors' => ['Jose', 'Carlos'] },