fix: improve external script plugin (#632)

Author: slengyel-lnx

lib/plugins/external-script.js

@@ -10,7 +10,7 @@ function handleExternalScript() {
       var newScript = document.createElement('script');
 
       Array.prototype.slice.call(script.attributes).forEach(function (attribute) {
-        newScript[attribute.name] = attribute.value;
+        newScript.setAttribute(attribute.name, attribute.value);
       });
 
       script.parentNode.insertBefore(newScript, script);