Feature/separate undef and false

Sometimes I want insert value `0` or `"0"` (it's real ZERO value not false :) with haml's `= PERL_EXP` syntax. But it is not possible right now since `Text::Haml` regard it as false and ignore.

Inserting Perl works well like:

```haml
  %div
    = undef || 0
```

But inlining it doesn't work:

```haml
  %div= undef || 0
```

Please check it and if this is not proper or doesn't make sense, just ignore it. :)