Unflatten method doesn't work with "toString" key

[yukienomiya]

Problem

The unflatten method doesn't work with a key named toString.

For example, starting with this object:

{
  "p5": {
    "y": "The y component of the vector",
    "toString": {
      "returns": "String: "
    }
  }
}

Then flatten it:

{
  'p5/y': 'The y component of the vector',
  'p5/toString/returns': 'String: '
}

And then unflatten it:

{
  "p5": {
    "y": "The y component of the vector"
  }
}

The toString key disappears.

Workaround

I ended up adding a prefix to all the keys while flattening the object (with the transformKey option), and then removing it after unflattening it again. To remove the prefix I used the map-obj library to map all the key and values from the original object to a new one, where the keys were substrings of the original ones.