Fix unexpected behavior of Lens.withDefault

[blacktaxi]

This tracks #17

The issue is that Lens.withDefault with unexpectedly use an undefined value in place of a value passed into a Lens.set when it is equal to the default value provided.

It makes sense, as it is more lawful (in lens laws terms) to remove a value when set to default. Although technically, the current implementation is not lawful either, because { a: 5, b: undefined } is not strictly equal to { a: 5 }.

I see these options:

1. Change Lens.withDefault to actually modify the data structure even when set to a default value.
2. Leave Lens.withDefault as it is and implement a separate lens with this new behavior.

Bonus option: if we decide to keep current Lens.withDefault, fix it to be truly lawful.