Add merged_by and merged_at fields to MergeRequest (#372)

* Support merged_by and merged_at for merge requests

* add support for a couple of fields on the MergeRequest: `merged_by` and `merged_at`

* format file using gofmt

merge_requests.go

@@ -68,6 +68,14 @@ type MergeRequest struct {
 	Milestone                 *Milestone `json:"milestone"`
 	MergeWhenPipelineSucceeds bool       `json:"merge_when_pipeline_succeeds"`
 	MergeStatus               string     `json:"merge_status"`
+	MergedBy                  struct {
+		ID        int        `json:"id"`
+		Username  string     `json:"username"`
+		Name      string     `json:"name"`
+		State     string     `json:"state"`
+		CreatedAt *time.Time `json:"created_at"`
+	} `json:"merged_by"`
+	MergedAt                 *time.Time `json:"merged_at"`
 	Subscribed               bool       `json:"subscribed"`
 	SHA                      string     `json:"sha"`
 	MergeCommitSHA           string     `json:"merge_commit_sha"`