Allow getting merge requests which would close an issue

Author: violuke

lib/Gitlab/Api/Issues.php

@@ -213,4 +213,14 @@ public function awardEmoji($project_id, $issue_iid)
     {
         return $this->get($this->getProjectPath($project_id, 'issues/'.$this->encodePath($issue_iid).'/award_emoji'));
     }
+
+	/**
+	 * @param int $project_id
+	 * @param int $issue_iid
+	 * @return mixed
+	 */
+	public function closedByMergeRequests($project_id, $issue_iid)
+	{
+		return $this->get($this->getProjectPath($project_id, 'issues/'.$this->encodePath($issue_iid)).'/closed_by');
+	}
 }