Fix Definition#name to convert @options[:as] to string

In case define property with `as` and `selectable`
options:

```
property :first_name, as: :name, selectable: true
```

Representer#handle_definition? returns false
because `@options[:as]` is a Symbol and selector keeps
their keys as String.
Reproduce property option bug

Can't work with selector when `as` and `selectable` were passed to
`property` method.
Merge pull request #13 from yui-knk/fix/representer

Fix Representer::Definition#name to always return as string