Update head-dedupe.js
Update head-dedupe.js

This reflects the latest version of the polyfill from MDN (adds configurable: true, writable: true).

This fixes an issue in IE11 where other libs try to extend Array (and implement .find) which leads to 
```SCRIPT5045: Assignment to read-only properties is not allowed in strict mode```

For reference:
https://github.com/mobxjs/mobx/issues/1028

Is it possible to do a release after merge so we can wipe our caches?
sanfrancesco/prerendercloud-ajaxmonkeypatch,sanfrancesco/prerendercloud-ajaxmonkeypatch