Fix the problem about not numerical value.

Hi,

MooX::LvalueAttribute does not work certainly if it is given not numerical value (e.g. string).
https://gist.github.com/moznion/6895043
The above code is the SYNOPSIS of MooX::LvalueAttribute, but it does not work.

The cause is Variable::Magic.
https://gist.github.com/moznion/6896660
Code references in `wizard()` expects the return value as numerical value.
However MooX::LvalueAttribute does not meet this condition.

So I fixed it.
Could you review and merge this?