Fix cache for string IDs that have exactly 12 bytes

* The method `isValid` from MongoDB just checks if a string has 12 bytes. But you could have a string ID which length=12 which would be unintentionally converted to an ObjectID which in returns causes an invalid cache miss.

Author: Jonas Meier

src/cache.js

@@ -10,7 +10,7 @@ const stringToId = str => {
     return str
   }
 
-  if (ObjectId.isValid(str)) {
+  if (ObjectId.isValid(str) && new ObjectId(str) === str) {
     return new ObjectId(str)
   }