ConfigObject is not immutable - can mutate through keySet()

[A248]

Despite the documentation, ConfigObject can be mutated. The key set of a map supports removal operations. This happens because of the following factors.

1. A library user calls ConfigValueFactory.fromMap.
2. ConfigImpl#fromAnyRef constructs SimpleConfigObject using new SimpleConfigObject(origin, values) where values is a HashMap.
3. SimpleConfigObject's keySet() method returns a mutable Set<String>. This Set cannot be added to, but it supports element removal.