[bug] wrong close time issue on milestone

gitea version: 1.11.0+dev-56-g28f60bb5c

test url: https://gitea.com/gitea/tea/milestones?state=closed
![Bildschirmfoto zu 2019-10-28 23-33-43](https://user-images.githubusercontent.com/24977596/67723123-6211d280-f9db-11e9-8a0f-f657ba89f2a9.png)
