Add tests to cover multiple attribute for grouped select in associations

Also, updates the CHANGELOG.md.

CHANGELOG.md

@@ -7,6 +7,7 @@
 * Allow custom error on forms without model. [@victorperez](https://github.com/victorperez)
 * Do not support Ruby < 2.3 anymore. [@gssbzn](https://github.com/gssbzn)
 * Add color input type. [@gssbzn](https://github.com/gssbzn)
+* Set multiple attribute for grouped selects also. [@ollym](https://github.com/ollym)
 
 ### Bug fix
 * Improve disabled option to input_field. [@betelgeuse](https://github.com/betelgeuse)

test/form_builder/association_test.rb

@@ -243,4 +243,10 @@ class AssociationTest < ActionView::TestCase
     assert_equal({ as: :check_boxes, collection_wrapper_tag: :ul, item_wrapper_tag: :li },
                  options)
   end
+
+  test 'builder with group select considers multiple select by default' do
+    with_association_for @user, :tags, as: :grouped_select, group_method: :group_method
+
+    assert_select 'select[multiple="multiple"].grouped_select'
+  end
 end

test/support/models.rb

@@ -76,7 +76,11 @@ Friend = Struct.new(:id, :name) do
   end
 end
 
-class Tag < Company; end
+class Tag < Company
+  def group_method
+    ["category-1"]
+  end
+end
 
 TagGroup = Struct.new(:id, :name, :tags)