Fix regression in set-constant scriptlet

Related issue:
- uBlockOrigin/uBlock-issues#947

`Object.prototype` is not an instance of `Object`, but
yet is still a JS object; thus the solution is to
additionally compare against `typeof`.

assets/resources/scriptlets.js

@@ -549,7 +549,7 @@
         const prop = chain.slice(0, pos);
         let v = owner[prop];
         chain = chain.slice(pos + 1);
-        if ( v instanceof Object ) {
+        if ( v instanceof Object || typeof v === 'object' ) {
             makeProxy(v, chain);
             return;
         }