Use GroupDetail for POST /groups

Feedback:
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1942#note_30680817

lib/api/groups.rb

@@ -104,7 +104,7 @@ module API
             )
           end
 
-          present group, with: Entities::Group, current_user: current_user
+          present group, with: Entities::GroupDetail, current_user: current_user
         else
           render_api_error!("Failed to save group #{group.errors.messages}", 400)
         end

spec/requests/api/groups_spec.rb

@@ -513,10 +513,8 @@ describe API::Groups do
 
             post api("/groups", admin), group
 
-            created_group = Group.find(json_response['id'])
-
             expect(response).to have_http_status(201)
-            expect(created_group.shared_runners_minutes_limit).to eq(133)
+            expect(json_response['shared_runners_minutes_limit']).to eq(133)
           end
         end
       end