adding PHP example for custom HTTP headers in documentation + improving logs for custom HTTP headers

build/docs/content/08-merge.md

@@ -50,6 +50,6 @@ $files = [
     DocumentFactory::makeFromPath('file2.pdf', 'file2.pdf'),
 ];
 $request = new MergeRequest($files);
-$dest = "result.pdf";
+$dest = 'result.pdf';
 $client->store($request, $dest);
 ```