Does not work with nil values.

I have different environments, and I need to define and environmental variable on one of those and not in others, nor of this seems to work for me:

```
DEBUGGING: ~
development:
  DEBUGGING: true

Figaro.env.debugging #=> ""
```

```
development:
  DEBUGGING: true

Figaro.env.debugging #=> "true"
```

I may be missing something.