features for batch_action form option

This commit was cherry-picked from #3140. The only change made by me was:

--- a/features/index/batch_actions.feature
+++ b/features/index/batch_actions.feature
@@ -155,7 +155,7 @@ Feature: Batch Actions
       """
       ActiveAdmin.register Post do
         batch_action :destroy, false
-        batch_action(:action_with_form, form: { type: -> { ["a", "b"] } }) {}
+        batch_action(:action_with_form, form: ->{ {type: ["a", "b"]} }) {}
       end
       """