Don't check for admin permission if we're not

going to change shared_runners_minutes_limit

Feedback:
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1942/diffs#note_30680494

lib/api/groups.rb

@@ -128,7 +128,10 @@ module API
         authorize! :admin_group, group
 
         # EE
-        authenticated_as_admin! if params[:shared_runners_minutes_limit]
+        if params[:shared_runners_minutes_limit].to_i !=
+            group.shared_runners_minutes_limit.to_i
+          authenticated_as_admin!
+        end
 
         if ::Groups::UpdateService.new(group, current_user, declared_params(include_missing: false)).execute
           present group, with: Entities::GroupDetail, current_user: current_user

spec/requests/api/groups_spec.rb

@@ -279,6 +279,14 @@ describe API::Groups do
 
         expect(response).to have_http_status(403)
       end
+
+      it 'returns 200 if shared_runners_minutes_limit is not changing' do
+        group1.update(shared_runners_minutes_limit: 133)
+
+        put api("/groups/#{group1.id}", user1), shared_runners_minutes_limit: 133
+
+        expect(response).to have_http_status(200)
+      end
     end
 
     context 'when authenticated as the admin' do