Fix evalScripts() (#186)

* Set the id of the inserted <script>.
* Check if the <script> still exists before trying to remove it.

lib/eval-script.js

@@ -12,6 +12,7 @@ module.exports = function(el) {
   }
 
   script.type = "text/javascript"
+  script.id = el.id;
 
   /* istanbul ignore if */
   if (src !== "") {
@@ -33,7 +34,7 @@ module.exports = function(el) {
   // execute
   parent.appendChild(script)
   // avoid pollution only in head or body tags
-  if (parent instanceof HTMLHeadElement || parent instanceof HTMLBodyElement) {
+  if ((parent instanceof HTMLHeadElement || parent instanceof HTMLBodyElement) && parent.contains(script)) {
     parent.removeChild(script)
   }