Missing field merged_at in MergeRequest

[lesiak]

Check https://docs.gitlab.com/ee/api/merge_requests.html

Unfortunately, it seems that the field is not exposed in the webhook event

[
  {
    "id": 1,
    "iid": 1,
    "project_id": 3,
    "title": "test1",
    "description": "fixed login page css paddings",
    "state": "merged",
    "merged_by": {
      "id": 87854,
      "name": "Douwe Maan",
      "username": "DouweM",
      "state": "active",
      "avatar_url": "https://gitlab.example.com/uploads/-/system/user/avatar/87854/avatar.png",
      "web_url": "https://gitlab.com/DouweM"
    },
    "merged_at": "2018-09-07T11:16:17.520Z",
    "closed_by": null,
    "closed_at": null,

[lesiak]

Added a merge request #412