Fix issue #222, issue assign is reset when it is updated/moved

Fix issue #222 (closed), assign reset in issue when updated.

This is a quick fix against gitlab api weirdness, cf https://gitlab.com/gitlab-org/gitlab-ce/issues/22664

Please merge this request, otherwise the current version is quite unusable.

merged

Hello @all. We release it on this week.

@Bethuys thank you for help;