ensure that the closed_at is set for closed

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: #5446
Signed-off-by: Roman <romaaan.git@gmail.com>

models/issue.go

@@ -325,6 +325,10 @@ func (issue *Issue) APIFormat() *api.Issue {
 		Updated:  issue.UpdatedUnix.AsTime(),
 	}
 
+	if issue.ClosedUnix != 0 {
+		apiIssue.Closed = issue.ClosedUnix.AsTimePtr()
+	}
+
 	if issue.Milestone != nil {
 		apiIssue.Milestone = issue.Milestone.APIFormat()
 	}