_type Attribute is now saved on inherited models.

Scenario:
```ruby
class Animal
  tenant :account
  field :name, type: String
end

class Cat < Animal
  field :age, type: Integer
end
```

When saving a ```Cat``` model, its ```_type```-attribute was not saved to mongodb. So when calling ```Animal.last``` the resulting object was of type ```Animal``` instead of ```Cat```.

Solved this by adding a call to ```super``` in ```define_singleton_method``` in the ```tenant```-Method.