Doesn't re-create the index if an index-setting (like `unique`) changes

First I created a property on an object and also created a unique index for it by adding `{ unique: true }`. After some time of development I had to remove it, but it still remains in MongoDB. After reading the manual, I knew that it works when adding `{ index: false }` to the configuration, but this doesn't help on a property where I want an index, but not unique.