gitlab issue event sends assignees (multiple) not assignee (single) (#161)

I also got hit by the gitlab API change where the IssueEventPayload
doesn't contain a field 'assignee' but only 'assignees' - even though
there's only one...

#159

The 'assignee' field is deprecated but not removed
(https://docs.gitlab.com/ee/user/project/integrations/webhook_events.html#issue-events)
so it's kept in the struct.

---------

Co-authored-by: Kuba Raczkowski <kuba.raczkowski@spectricity.com>

Author: kubaraczkowski

gitlab/payload.go

@@ -38,6 +38,7 @@ type IssueEventPayload struct {
 	Repository       Repository       `json:"repository"`
 	ObjectAttributes ObjectAttributes `json:"object_attributes"`
 	Assignee         Assignee         `json:"assignee"`
+	Assignees        []Assignee       `json:"assignees"`
 	Changes          Changes          `json:"changes"`
 }
 

testdata/gitlab/issue-event.json

@@ -77,5 +77,10 @@
     "name": "User1",
     "username": "user1",
     "avatar_url": "http://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=40\u0026d=identicon"
-  }
+  },
+  "assignees": [{
+    "name": "User1",
+    "username": "user1",
+    "avatar_url": "http://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=40\u0026d=identicon"
+  }]
 }