this.data does not have closed property

gitlab api does not have ```closed``` property in the json data.
```state``` property contains either ```closed``` or ```opened``` value instead.
Please, refer https://docs.gitlab.com/ee/api/issues.html

Author: hkmoon

src/models/issue.js

@@ -71,7 +71,7 @@ class issue extends hasTimes {
     }
 
     get closed() {
-        return !!this.data.closed;
+        return this.data.state === 'closed';
     }
 
     get updated_at() {
@@ -111,4 +111,4 @@ class issue extends hasTimes {
     }
 }
 
-module.exports = issue;
+module.exports = issue;