make identifier regex in makeKeyGeneric more restrictive

makeKeyGeneric only allows letters  as part of an identifier in $[<identifier>]

lib/mongo-object.js

@@ -681,7 +681,7 @@ export default class MongoObject {
    */
   static makeKeyGeneric(key) {
     if (typeof key !== 'string') return null;
-    return key.replace(/\.([0-9]+|\$\[.*\])(?=\.|$)/g, '.$');
+    return key.replace(/\.([0-9]+|\$\[[a-zA-Z]*\])(?=\.|$)/g, '.$');
   }
 
   /** Takes a string representation of an object key and its value