Check the type of the default

Hi,

I think the default option should match the type.  This is a problem I ran into while using Thor.  I had code like this:
```ruby
method_option :foo, :aliases => '-f', :default => '50', :type => :numeric
```

I expect the default value `'50'` to be a numeric, but of course it was a string. 

Hopefully, this pull request solves the problem. ;-)

Cheers,
Huw.