Fix utils#valuesAreObjectIds

Currently `utils.valuesAreObjectIds` returns true, if you pass in a collection object as `item` and an ObjectId as `value`. The following call of `item.equals(value)` in the function `utils.matchValues` then raised an exception in one case in my code.

Using the proposed implementation of `utils.valuesAreObjectIds` the exception doesn't get raised and all Mockgoose tests still pass.