fix(BaseManager): BaseManager#valueOf should return cache (#3776)

* BaseManager#valueOf should return cache

* Update Util#flatten to handle valueOf being a Collection

* Update Util.js - typo

Co-Authored-By: Amish Shah <amishshah.2k@gmail.com>

Co-authored-by: Amish Shah <amishshah.2k@gmail.com>

src/managers/BaseManager.js

@@ -72,6 +72,10 @@ class BaseManager {
     if (typeof idOrInstance === 'string') return idOrInstance;
     return null;
   }
+
+  valueOf() {
+    return this.cache;
+  }
 }
 
 module.exports = BaseManager;