dont lose falsy values

Falsy values like `''` and `0` are replaced with `undefined` when `.get()` is called for them. This fix remedies that
Model values that arent undefined returned

Model values can be falsy but not undefined. A `null`, `''` or `0` value are real values that can be stored with `.set` but, previously, couldnt be retrieved. This remedies that.
add falsy test. update change for delegate pass-through to undefined
Merge pull request #150 from yanatan16/patch-1

Don't lose falsy values in model