[VarDumper] Fix JS to expand / collapse

Author: nicolas-grekas

src/Symfony/Component/VarDumper/Dumper/HtmlDumper.php

@@ -167,9 +167,9 @@ protected function getDumpHeader()
     };
 
 refStyle.innerHTML = 'pre.sf-dump .sf-dump-compact, .sf-dump-str-collapse .sf-dump-str-collapse, .sf-dump-str-expand .sf-dump-str-expand { display: none; }';
-(doc.documentElement.firstElementChild || doc.documentElement.children[0]).appendChild(refStyle);
+doc.head.appendChild(refStyle);
 refStyle = doc.createElement('style');
-(doc.documentElement.firstElementChild || doc.documentElement.children[0]).appendChild(refStyle);
+doc.head.appendChild(refStyle);
 
 if (!doc.addEventListener) {
     addEventListener = function (element, eventName, callback) {