allow_nil option transforms nil values when should not

Hi!

With #27 and its release in v0.9.5 a bug was introduced: `nil` values are converted to `0`, `0.0` or `''` if `allow_nil` is not allowed.

I think that the behavior should be that a nil should be allowed if option is enabled, but if not, the value should remain.

So, in the `Integer` case, I would suggest:

```ruby
case value
when nil then options[:allow_nil] ? nil : value
...
```