Feature request: add ability to prevent modifying certain value in `unflatten`

[jamauro]

I have a use case where I'd like to prevent the target's value from being modified in unflatten. There's probably a number of ways to approach it but here's what I was thinking:

// prevent modifying any object with a value of 'testValue'
unflatten(obj, {filter: {value: 2}})

// prevent modifying any object with a key of 'thing'
unflatten(obj, {filter: {key: 'thing'}})

// prevent modifying the object {thing: 2}
unflatten(obj, {filter: {key: 'thing', value: 2}})

[jonshipman]

Common way to reproduce, if you use mongodb and have a bunch of generated ObjectIds as values, they will be stripped out into new objects, e.g. {}

[mdwekat]

happened to me when unflatten object with mongoId the ids will end up as {}