fix #425 edit issue accepts an array of assignees like create (#426)

Ref: https://docs.gitlab.com/ce/api/issues.html#edit-issue

issues.go

@@ -310,7 +310,7 @@ type UpdateIssueOptions struct {
 	Title            *string    `url:"title,omitempty" json:"title,omitempty"`
 	Description      *string    `url:"description,omitempty" json:"description,omitempty"`
 	Confidential     *bool      `url:"confidential,omitempty" json:"confidential,omitempty"`
-	AssigneeID       *int       `url:"assignee_id,omitempty" json:"assignee_id,omitempty"`
+	AssigneeIDs      []int      `url:"assignee_ids,omitempty" json:"assignee_ids,omitempty"`
 	MilestoneID      *int       `url:"milestone_id,omitempty" json:"milestone_id,omitempty"`
 	Labels           Labels     `url:"labels,comma,omitempty" json:"labels,omitempty"`
 	StateEvent       *string    `url:"state_event,omitempty" json:"state_event,omitempty"`