Just check the value, simple and stupid

Feedback:
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1942#note_31511130

lib/api/groups.rb

@@ -128,14 +128,11 @@ module API
         authorize! :admin_group, group
 
         # EE
-        if params[:shared_runners_minutes_limit]
-          group.shared_runners_minutes_limit =
+        if params[:shared_runners_minutes_limit].present? &&
+            group.shared_runners_minutes_limit.to_i !=
               params[:shared_runners_minutes_limit].to_i
-
-          if group.shared_runners_minutes_limit_changed?
           authenticated_as_admin!
         end
-        end
 
         if ::Groups::UpdateService.new(group, current_user, declared_params(include_missing: false)).execute
           present group, with: Entities::GroupDetail, current_user: current_user