ensure that the `closed_at` is set for closed (#5450)

ensure that the `closed_at` is set for closed (#5450)

right now the `closed_at` field for json responses is not filled during
the `APIIssue` creation for api responses.

For a closed issue you get a result like:
```json
"state":"open","comments":0,"created_at":"2018-11-29T16:39:24+01:00",
"updated_at":"2018-11-30T10:49:19+01:00","closed_at":null,
"due_date":null,"pull_request":null}
```
which has no information about the closing date. (which exists in the
db and ui)
with this PR the result changes to this:

```json
:null,"assignee":null,"assignees":null,
"state":"closed",
"comments":0,"created_at":"2018-11-29T16:43:05+01:00",
"updated_at":"2018-12-02T19:17:05+01:00",
"closed_at":"2018-12-02T19:17:05+01:00",
"due_date":null,"pull_request":null}
```

fixes: https://github.com/go-gitea/gitea/issues/5446
Signed-off-by: Roman <romaaan.git@gmail.com>