change exec context for batch_action form option

fix for a https://github.com/activeadmin/activeadmin/pull/1815#issuecomment-59659670
I found way. Is because HashWithIndifferentAccess converts all keys to strings thats needed by https://github.com/activeadmin/activeadmin/blob/fix_batch_action_context/lib/active_admin/batch_actions/controller.rb#L10. In a current local version I moved the converting to there.