MWS.orders.next don`t clear last request next_token

In [api/base.rb:50](https://github.com/elyngved/ruby-mws/blob/master/lib/ruby-mws/api/base.rb#L50).

If i need 2 request to get all orders, after success run one times `mws.orders.next` even the last request do not return a 'next_token' the `@next[:token]` will persist the last request`s token, code like below will loop forever.

```ruby
save_or_update_orders(response.orders)
while mws.orders.has_next?
  puts 'into next page...'
  save_or_update_orders(mws.orders.next.orders)
end
```

Because if next reponse do not have `next_token` then 
```ruby
if @response.respond_to?(:next_token) and @next[:token] = @response.next_token
```
will not execute `@next[:token] = ...`