Do not try to set attribute for not selected attributes

Using enumerize 0.8.0 and Rails 3.2.17

```ruby
class User
  extend Enumerize

  enumerize :sex, :in => [:male, :female]
  enumerize :role, :in => [:user, :admin]
end

User.create!(:sex => :male)
User.select(:id).first.attributes.keys
=> ["id", "role", "sex"]
```

Looks like enumerize try to set attribute that is not selected.

so, I had modified to skip to the setting default value when `ActiveModel::MissingAttributeError` occurs.