The parent commit appears to be totally untested.

The placement of the method makes no sense, and the API route is wrong.
The parameter is missing, and the expected response type is wrong.

src/main/java/org/kohsuke/github/GHPullRequest.java

@@ -205,4 +205,16 @@ public class GHPullRequest extends GHIssue {
         };
     }
 
+    /**
+     * Merge this pull request.
+     *
+     * The equivalent of the big green "Merge pull request" button.
+     *
+     * @param msg
+     *      Commit message. If null, the default one will be used.
+     */
+    public void merge(String msg) throws IOException {
+        new Requester(root).method("PUT").with("commit_message",msg).to(getApiRoute()+"/merge");
+    }
+
 }