Fix incorrect namespace conversion.

Fix for issue https://github.com/pwnieexpress/snapi/issues/3

Btw, what do you expect from method `namespace` for classes with same name defined in different modules?

For example:

``` ruby
require 'snapi'

module A
  class ClassName
    include Snapi::Capability
  end
end

p A::ClassName.namespace
p Snapi.capabilities

module B
  class ClassName
    include Snapi::Capability
  end
end

p B::ClassName.namespace
p Snapi.capabilities
```

Output:
```
:class_name
{:class_name=>A::ClassName}
:class_name
{:class_name=>B::ClassName}
```

Looks like a bug.